Article Brakes - Operation of the Master Cylinder

[jfheath]

The operation of a hydraulic master cylinder - as used for the front brake lever, rear brake pedal, clutch lever and SMC. Coloured diagrams and explanations.


1 - 'Relaxed' Brake Master Cylinder (ie Brakes are not Applied)



The image shows the front master cylinder and the reservoir immediately above it - just like the ones used on the clutch and brake lever on the ST1300. The piston and push rod are shown in grey.
The primary seal (blue) is the one that applies considerable pressure to the braking system.
The secondary seal (black) maintains a body of fluid behind the primary seal which is gravity fed from the reservoir above. It also isolates the system from the outside world. This seal is never normally put under any great (ie braking force) pressure. The ST1300s secondary master cylinder being an exception to this arrangement - being fed from the rear pedal master cylinder.
Note the larger inlet port between the reservoir and the piston/cylinder. This maintains a volume of fluid between the two seals, fed constantly from the reservoir. The inlet port is never closed off by the position of the piston and seals.
The tiny compensating port is immediately in front (just to the left) of the primary seal. This allows fluid in the braking system to return into the reservoir and relieves any build up of pressure when the braking system is in this 'relaxed' state.
The spring is responsible for returning the piston to the right on this diagram. The push rod at the right hand end is operated by the brake lever.
The hose to the brake calliper is shown descending beyond the bottom of the picture on the left of the master cylinder.
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2 - Master Cylinder when Brake is Applied



In Diagram 2, the piston has been pushed to the left. Fluid under pressure (pink) is forced down the brake line to the pistons in the brake calliper. Note that the primary seal (blue) has moved past the compensating port, preventing any fluid from returning to the reservoir. The fluid under pressure is responsible for pushing out the pistons in the brake calliper which push the brake pads onto the disc rotors. Once the brake pads are in contact with the disc rotors it takes only a small movement in the master cylinder (brake lever) to exert a force sufficient to bring the bike to a halt.
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3 - Releasing the Brake Lever
(now showing the correct diagram - thanks Midlife)



When the brake lever is released, the piston and plunger are returned to the initial 'relaxed' position primarily as a result of the spring pressure in the master cylinder, but aided by the flexible hoses and the calliper piston seals returning to their normal state.
Each time the brakes are applied, the pads wear down a tiny fraction, and the calliper pistons are pushed out further than they were before the brakes were applied. When the brakes are released, the spring returns the master cylinder piston to its relaxed state and a negative pressure (vacuum) is created in the brake lines. Fluid is able to flow past the lips of the primary seal to allow for the fact that the pads have worn down a little.
The primary seal yields easily as the piston moves back - behaviour that is perfectly normal in exactly the same way that the seal in a bicycle pump gives way to allow air to get back into the tube on the upstroke.
The diagram shows the piston towards the end of its travel back to its 'relaxed' state, and the red arrow indicates the flow of extra fluid from the yellow reservoir fluid and into the brake lines (green fluid).

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4 - Dealing with Pressure Build-up



When the brake is not being applied, and the system is in a relaxed state, it is important that fluid is able to 'flow' to and from the reservoir as required. This is can happen when:-
* A slight warps in the disc rotors or general chattering of the brakes over bumpy surfaces pushes the pistons in slightly.
* An increases in temperature causes the fluid to expand.
* The bike cools down in the garage overnight and the fluid contracts. The relief port allows fluid to flow into the brake lines rather than suck the calliper pistons back.
* The brake pads are replaced and the calliper pistons are pushed in. The displaced fluid returns to the reservoir through the tiny relief port. No damage can result in pushing in the calliper pistons in this way if the system has been flushed with new fluid and the exposed calliper pistons are clean. Otherwise it is better to expel the old fluid from the calliper bores via the bleed valve.
The picture shows the pressure in the brake line being allowed to pass through the tiny compensating port into the reservoir, once the brake lever has been released. To prevent the 'fountain' shown in the diagram, a small chromed disc clips slightly above the port at the bottom of the reservoir.
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5 - Blocked Pressure Relief Port



If the piston is prevented from returning to its proper 'relaxed' state, then the Primary Seal will not expose the tiny compensating port. This could be caused by corrosion behind the piston, as shown. This part of the piston is open to the elements unless treated with silicone grease and covered with a rubber boot.
Alternatively, the compensating port itself could be blocked. Fluid that isn't replaced every year can turn into a thick gel which accumulates in the bottom of the reservoir. Or perhaps debris has fallen into the reservoir during a service.
Whatever, if the compensating port is not clear, or the piston fails to return properly to expose the compensating port, pressure builds up (red) and the brakes lock on solid. I have seen one situation recently where a master cylinder service kit was supplied incorrectly. The push rod was slightly too long and front brakes locked on solid at the first application and would not release.
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The illustrations refer to the ST1300 front master cylinder, but the principles also apply to the secondary master cylinder and the rear master cylinder, even though their relief port is connected to the fluid inlet line rather than to the reservoir itself.
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Final note. Throughout this document, I have made much more of the role of the compensating port in relieving pressure than its other functions.
In fact, the port will allow fluid to flow in either direction to add or remove fluid from the lines to compensate for the fact that the existing fluid will expand or c
ontract as the temperature fluctuates. To do this, the master cylinder piston has to be returned to its relaxed position so that the port is not blocked by the seal. The spring in the master cylinder ensures that this happens.

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Edit July 2020

There is an excellent discussion below, from which I accientally removed my contributions in a cleanup operation. Don't ask! Fortunately, other members have quoted my missing replies, so the gist of that discussion still makes sense If those quotes are read.

 

[MidLife]

Some basic information here about the operation of the brake master cylinder.

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3 - Releasing the Brake Lever

When the brake lever is released, the spring pushes the piston and plunger back to its initial 'relaxed' position. The diagram shows the piston at the start of its return to the 'relaxed' state. The fluid in the brake line (green) is no longer under pressure. As the brake pads wear down, the pistons in the calliper will need to push out further and this requires more fluid. As the MC piston is pushed to the right, by the spring, the fluid behind the primary seal (yellow) is able to flow past the outer edge of the seal to take up the space created by the extended pistons.

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Thanks for putting this together!

A quick comment on (3). The piston is shown in the same position as in (2), so although the lever is released, the pressure in the line is still the same as in (2). There is no back flow across the front seal as the line pressure is higher than the reservoir pressure and the seal is still energized (remains energized until it travels back across the port). The spring does push the piston back, but initially the pressure (up to about 2000 psi) does most of the work. The spring is essential to insure the seal completely clears the port once the seal reaches the port and the line pressure equalizes with the reservoir pressure and there is no pressure differential left across the seal to drive the piston back. And then the spring insures the piston stays back, which is critical as illustrated by your subsequent schematics.

 

[MidLife]

The returning pressure when the lever is released comes from the calliper piston seals returning from their deformed state and from the limited compressibility of the fluid itself.

When braking, the line pressure starts building up as soon as the front seal passes the compensating port. With no leak, whatever pressure was built up while braking will push the piston back. As you say, pad wear for a single brake application will not cause noticeable oil volume change, the heat from hard braking however can cause fluid expansion which will raise the pressure. Volume expansion is made up for once returning over the compensating port.

Line expansion, as you know, also plays a role in how much travel it takes for building up pressure (the relatively soft lines act as an accumulator bladder), and can be reduced by installing stiffer reinforced lines.

Fluid compressibility is limited but far from negligible. Especially considering that even brand new fluid already contains some air and air content increases with usage.

I suppose you have taken a ST 1300 master cylinder apart? Am curious about the orientation of the secondary seal. In most applications, such a seal would be oriented in the opposite direction of the primary seal (opposite of shown in the schematic), to keep the crud out. At times, even a different type seal is used as secondary.

Or are you saying that fluid never flows past the primary seal and that the fluid is replenished via the relief port?

The "relief port" is often called "compensating port", as it allows oil to flow both ways to compensate for volume changes due to temperature changes for instance and other effects that you have described.

I can of course not say that fluid will "never" flows past the primary seal, as I know too little about the system.

I can only say that in normal circumstances, I see no obvious reason for the fluid to flow over the seal.

Take the case where you grab the brake while standing still: Whatever is compressed in will expand back out when releasing the lever and there is no need for flow by.

On a single application while riding, the volume change from pad wear would be so small that it would not warrant flow by either.

On a longer ride with frequent applications, the reservoir can be viewed as acting similarly to the coolant reservoir. Oil will expand in the reservoir through the compensating port as it heats up and then flow back into the system as it cools down after the ride, back down to a level where it compensates for all the wear that occurred during the ride.

One possible case where oil would need to flow by the seal could be if there is leakage in the system. The spring would push the piston back when releasing the lever and create a slight vacuum in the line which would call for flow by to allow the piston to travel all the way back (which could explain why the hole between the two seals is larger, to prevent too restrictive of a flow in case flow back is required to compensate for a leak. This would allow quick pumping of the brake to overcome the leak and come to a stop).

 

[MidLife]

Your insight has been useful

More like trying to keep up with all the thoughts that lead to your instructive article.

Looking forward to your upcoming SMC article.

 

[Anna'sDad]

Hello jfheath,

I see you've been busy studying the St1300 brake system, it is an interesting system!

I attempted to comment on your post, but I may have dawdled too long, as I can't seem to Quote from the original. No worries I copied & saved it!

1 - 'Relaxed' Brake Master Cylinder (ie Brakes are not Applied)

The primary seal (blue) is the one that applies considerable pressure to the braking system. The secondary seal (black) prevents leakage to the outside and water / air ingress. This seal is never put under any great pressure.

I believe this to be in error: The secondary seal is a component of the recuperation phase of master cylinder operation, it is not a weather seal.

Note the larger inlet port between the reservoir and the piston/cylinder. This keeps the space between the two seals full of fluid and is a gravity feed. The tiny relief port is immediately in front (just to the left) of the primary seal. This allows fluid in the braking system to return into the reservoir and prevents a build up of pressure when the brake is in this 'relaxed' state.

Identifying these two inlet ports can get confusing very quickly. The smaller port is commonly named: Bypass port, Intake & Return port, and Compensating port. Where as the larger port is commonly named: Intake port, Equalization port, Feed port, Replenishing port, and Recuperation port. In an attempt to reduce confusion, I've tried to use the terms Compensating port, and Recuperation port exclusively, when discussing the operation of Honda master cylinders.

3 - Releasing the Brake Lever

When the brake lever is released, the piston and plunger are returned to the initial 'relaxed' position. Initially this is from residual fluid pressure and from the residual energy stored by the deformation of the piston seals. Each time the brakes are applied, the pads wear down a tiny fraction, and the pistons may remain further out than they were before. The spring ensures that the master cylinder piston is returned all of the way so that the relief port is cleared - and that it stays there until the brakes are applied again.
The diagram shows the piston towards the end of its travel back to its 'relaxed' state. As the spring takes over, the fluid in the brake line (green) is will no longer be under any pressure, and fluid from the right (yellow) can flow past the outer edge of the primary seal to take up the space created by the extended calliper pistons.

The spring can (and does) push the master cylinder piston back faster than brake fluid can return from the caliper(s), which is due to numerous fluid resistances in the brake system (small internal bore of hydraulic piping, tiny hydraulic ports, delay valve, proportional valve). The effect of the master cylinder piston moving quickly is that fluid pressure drops in the chamber, creating a negative pressure (vacuum), the vacuum draws fluid around the primary seal from the chamber between the primary seal, and the secondary seal (this is the recuperation phase). Without the recuperation elements in place the vacuum would draw in air, and other contaminates from the outside.

4 - Dealing with Pressure Build-up

* The brake pads are replaced and the calliper pistons are pushed in. The displaced fluid returns to the reservoir through the tiny relief port.

This probably is not the best way to re-position caliper pistons when replacing brake pads. The caliper (usually) is the lowest component in the hydraulic system, over time debris and contaminates in the fluid will have settled in the caliper, stirring that sediment up by pushing in the pistons, and then forcing the dirty fluid back towards the master cylinder seems undesirable. A better method is to open the bleed screw, wasting the brake fluid as the caliper pistons are re-positioned. Then topping up the master cylinder with fresh brake fluid.

5 - Blocked Pressure Relief Port

If the piston is prevented from returning to its proper 'relaxed' state, then the Primary Seal will not expose the tiny relief port. This could be caused by corrosion behind the piston, as shown. This part of the piston is open to the elements unless treated with silicone grease and covered with a rubber boot.

This part of Honda master cylinder design appears to be the 'Achilles Heel' for the ST1300. Each master cylinder (there are three) provides minimal protection from the elements with merely a rubber bootie covering the open end of the master cylinder bore. Amazingly the rubber bootie seems to be adequate for both the hand & foot operated master cylinders, with only the occasional complaint of problems. However the location & positioning of the SMC (secondary master cylinder) serves to high-light just how woefully inadequate the rubber bootie is in protecting this master cylinder from the elements, as evidenced by the large quantity of complaints of corrosion in the SMC bore.

 

[Anna'sDad]

Hello MidLife,

I was reading your comments re jfheath's tome on ST1300 brakes, when a couple of items caught my attention.

There is no back flow across the front seal as the line pressure is higher than the reservoir pressure and the seal is still energized (remains energized until it travels back across the port).

Sorry, I believe this is not correct; due to the systemic resistances to fluid flow, the master cylinder piston spring will move the piston faster than the flow of returning fluid from the caliper can respond. This action will create a negative pressure in the master cylinder bore, this difference in pressure will force fluid to flow from the recuperation port, slipping past the primary seal, and filling the area between primary & secondary seals. When the master cylinder piston has returned to it's resting position, and reveals the compensating port, the slow returning brake fluid (as well as the 'additional' fluid that slipped past the primary seal) will return to the reservoir by way of the compensating port.

The spring does push the piston back, but initially the pressure (up to about 2000 psi) does most of the work.

The brake system pressure rises after all brake caliper piston motion has essentially stopped, continued application of the brake lever attempts to compress the fluid, which causes system pressure to increase, in turn applying the brake pads to the brake rotor. At this point it takes very little movement of the master cylinder piston to generate the high fluid pressures necessary to bring the vehicle to a stop. Inversely, when the brake lever is released the high system fluid pressures diminish rapidly before any appreciable movement of the master cylinder piston is noted, this lack of sustained high fluid pressure causes the flow rate of returning caliper fluid to slow substantially.

The spring is essential to insure the seal completely clears the port once the seal reaches the port and the line pressure equalizes with the reservoir pressure and there is no pressure differential left across the seal to drive the piston back.

Agreed, however the pressure differential is a negative pressure in the master cylinder bore, with atmospheric pressure on the reservoir the greater of the two.

Over the years of my ST1300 ownership, I've disassembled most of the ST1300 brake components (some of which will never go back together again) in my quest to fully comprehend how this system was designed to operate. I have also generated a number of sketches of the various components to aid in understanding this brake system, most of those drawings are available here on st-owners.com for anyone to view.

Cheers, Anna'sDad

 

[Anna'sDad]

I'm wondering if one of us has the terms 'Primary Seal' and 'Secondary Seal' the wrong way round ?

That would be me.

Seems I got my primaries & secondaries wrong way round in my response to both your 3 - Releasing the Brake Lever topic, and Mr. Midife's There is no back flow across the front seal as ... topic as well.

I've made the appropriate corrections where noted.

Thanks for keeping me on my toes.

 

[MidLife]

Hello MidLife

The brake system pressure rises after all brake caliper piston motion has essentially stopped,

Cheers, Anna'sDad

If the brakes function properly, there is practically no brake piston motion and hence no oil flow. The master essentially just increases the pressure in the line.

 

[MidLife]

To put figures on this, for every 0.1 mm of friction material worn off the pads, the calliper pistons have to move 0.2mm (pads wear evenly on both sides). This would require 3.2mm of movement at the master cylinder piston. The amount of fluid flow in the master cylinder starts to look a lot more significant ! (Assuming 10,000 miles for brake pads, 0.1mm would represent the amount of wear in 200 miles).

PM me if you want the mathematics for this.

My simplistic approach:

By the time the pads are worn out, the fluid in the reservoir will have dropped by let's say about 2 cu-in.

Let's also say that it takes about 10'000 brake applications to get there: this equate to an average of only .03 mm of master cylinder piston motion per brake application to pump this volume in the caliper pistons (using the 12 mm figure for the master piston diameter that you indicated above). Compensating for this .03 mm on the return stroke will hardly require any oil flowing around the primary.

Even an enthusiastic rider wearing the pads after 1000 applications would only cause the piston motion to travel an extra .3 mm to make up for wear.

If you are not convinced that fluid can pass the primary seal from behind......

You convinced me. No doubt the fluid has to be able to pass the seal, otherwise the Grand Prix riders you mentioned would not be able to pump the caliper pistons back out when needed and a slow leaking worn out older primary seal would not be able to pop right back when the lever is released.

Whatever the truth in that last section, the key point is that when the brake lever is released, the master cylinder piston springs back to its relaxed position. In doing so, the fluid in the lines and the calliper cannot keep up with the speed of the master piston.....

Sure.

But again, in normal braking conditions, how many of us just let go of the lever?

And then, when compressing the oil to apply the brakes, the response of the front feels pretty darn instantaneous. Not sure what would cause a delay on the decompression cycle.

One day, if one of us has nothing better to do, we could try assembling a cylinder without the spring, to get a feel for how noticeable this oil decompression delay might be.

 

[MidLife]

In an imaginary system where the primary seal does not flex to allow fluid to pass from behind (ie it is a perfect seal):-

What would happen to the calliper pistons when the brake lever is released ?
What would happen to the calliper pistons and the master piston when the brake lever is released after the pads have worn down a bit ?
How would extra fluid get into the braking system to compensate for the pad wear ?

Please do not read too much in the comment I was trying to make. I did not intend for it to be a distraction from your impressive body of work (am in fact browsing your brilliant SMC postings trying to figure out what could be wrong with my own SMC!. Honda never did.)

I was just attempting to make the point that the master cylinder mainly compresses the fluid in the lines, rather than moving oil and creating a flow of oil in the lines. It is more like compressing a hydraulic spring, rather than oil flowing back and forth.

As discussed, a bidirectional ("perfect") seal would not be a good idea. It would spell murder in the Grand Prix case you mentioned and same when the seal gets older and wears out and allow for seepage. The master piston couldn't go back to its initial position and the brake would become inoperative.

But in normal brake operations, the volume it takes to compensate for the wear of one brake application is very small (less than 1/10th of a drop). So when the "perfect" seal moves back from this one brake application, it would hardly notice this missing 1/10th of drop in the line and would still move back past the port, at which time this missing 1/10th of a drop can be compensated for.

 

[MidLife]

..... the compensation port - how tiny it is .......

Typically, seals do not like to travel over ports. The easiest solution to prevent damage to the primary seal is to size the hole as small as possible (but not so small that it plugs too easily).

Didn't you mention actual size once in one of your posts?

.... the compensation port - how very close it is to the lip of the primary seal

The pressure in the line builds up as soon as the piston starts moving, which will at first act to waste and force some oil back into the reservoir through the compensation port until the seal is past the port. So the seal being as close as possible to the port helps minimize the initial part of the piston stroke that is partially "wasted".

The small size of the port is helping here: First in keeping the "wasted" portion of the stroke to a minimum (keeping the length of stroke it takes to fully cover the port to a minimum) and then to throttle back on the oil that is pushed back into the reservoir, so that the loss of pressure while the seal moves to cover the port is minimized as well (which is also why a "perfect" seal would still be able to make it back over the port on the return stroke).

The force we apply on the brake lever is primarily to compress the oil, but also to overcome seal friction in addition to compressing the spring. During "casual" braking, the pressure generated in the brake lines exerts over 100 lb of force pushing back on the piston, which is plenty to motivate the piston to start moving back. But this force is declining fast towards the end of the return stroke, as the oil decompresses, hence the importance of the spring to assist overcoming the seal friction towards the end of the travel and insure the piston moves all the way back, past the port.

 

[CYYJ]

John:

Do you know if it is possible to remove the small metal "flap" that covers both the compensating port (left one in your diagram) and the larger primary port, in order to check and see if any debris might be blocking the compensating port?

Michael

Flap

 

[CYYJ]

Thanks John, very kind of you to reply so quickly. I'm having problems priming the clutch system in my ST 1100, after having replaced the slave cylinder - I think I will remove that little part just to make sure that nothing is blocking flow into the compensating port.

Michael

 

[Igofar]

Michael, when you remove the deflector tab, you'll find a larger hole underneath it, and a very tiny pin size hole directly under the shield. This is where the fluid returns into the master cylinder housing. Take a small children's asperator, or a small syringe, fill which ever you use with clean brake fluid, then gently force the fluid into each of these holes without blowing air into them.
This will clear any debris out that may be in this area. Sometimes when the return hole is clogged, I've gently used a high E guitar string to open it back up again.
When you replace the deflector, be sure you place it back in the same direction it is in the picture, as it is possible to insert it upside down and block/restrict the opening, which will cause issues.
Larry

 

[paulcb]

Michael, I would also suggest that you might need to bleed the clutch MC at the banjo fitting outside the reservoir. I've had to do this on brakes.

It's obviously messy, so put some towels around everything and cover it as you squeeze the lever. Do this just like you would at the slave nipple... slightly open banjo, squeeze lever, close banjo, release lever. If you do this, I'd bleed it again down at the slave after the banjo, as you may have pushed some air down the line.

Maybe John or Larry will comment on the necessity of this.

 

[CYYJ]

Here are some pictures that show the "compensating port" (the smaller hole on the left in John's schematics in post #1).

I apologize for the quality of the pictures - I could not get the camera to focus on the tiny little hole, but hopefully between the three of them you will be figure out where the hole is.

I'm pretty sure that the purpose of the round cap above the compensating port is to prevent fluid from squirting up through this tiny hole when the cover is off the master cylinder and someone is working further downstream on the system, for example, pressing pistons back into place when changing brake pads.

Michael

Compensating Port

 

[CYYJ]

Michael, I would also suggest that you might need to bleed the clutch MC at the banjo fitting outside the reservoir.

Bleed the MC's out FIRST and you will be surprised at how easy it is to finish up the entire line.

PaulCB, ST-ABSU:

Thank you VERY MUCH for that suggestion - it solved my problem immediately.

I cracked the banjo bolt fitting on the clutch master cylinder, then pulled in the clutch lever, and a whole bunch of air came out - almost like a mousse. I repeated this twice (same technique as if I was bleeding down at the bleeding port, closing the bolt before releasing pressure), and then after having done this twice, I watched a whole stream of tiny air bubbles come out of the compensating port... it was like watching bubbles in a glass of Coca-Cola or champagne.

After about 60 seconds, the bubble stream from the compensating port into the master cylinder subsided, and I felt strong resistance when I pulled in the clutch lever. I was then able to complete the rest of the clutch bleed in no more than 5 minutes time.

I am very grateful to all of you, and also to John Heath, for your advice. I'm now back on the road again.

Michael

 

[paulcb]

Hey, that's great to hear Michael! Glad you're up and running.

We're still waiting on your trip report!

 

[Carl62566]

Thanks for putting this together!

A quick comment on (3). The piston is shown in the same position as in (2), so although the lever is released, the pressure in the line is still the same as in (2). There is no back flow across the front seal as the line pressure is higher than the reservoir pressure and the seal is still energized (remains energized until it travels back across the port). The spring does push the piston back, but initially the pressure (up to about 2000 psi) does most of the work. The spring is essential to insure the seal completely clears the port once the seal reaches the port and the line pressure equalizes with the reservoir pressure and there is no pressure differential left across the seal to drive the piston back. And then the spring insures the piston stays back, which is critical as illustrated by your subsequent schematics.

I'm new to this

Some basic information here about the operation of the brake master cylinder.


1 - 'Relaxed' Brake Master Cylinder (ie Brakes are not Applied)

View attachment 229794

The image shows the front master cylinder and the reservoir immediately above it - just like the ones used on the clutch and brake lever on the ST1300. The piston and push rod are shown in grey.

The primary seal (blue) is the one that applies considerable pressure to the braking system.

The secondary seal (black) maintains a body of fluid behind the primary seal which is gravity fed from the reservoir above. It also isolates the system from the outside world. This seal is never normally put under any great (ie braking force) pressure. The ST1300s secondary master cylinder being an exception to this arrangement.

Note the larger inlet port between the reservoir and the piston/cylinder. This maintains a volume of fluid between the two seals, fed constantly from the reservoir. The inlet port is never closed off by the position of the piston and seals.

The tiny compensating port is immediately in front (just to the left) of the primary seal. This allows fluid in the braking system to return into the reservoir and relieves any build up of pressure when the braking system is in this 'relaxed' state.

The spring is responsible for returning the piston to the right on this diagram. The push rod at the right hand end is operated by the brake lever.

The hose to the brake calliper is shown descending beyond the bottom of the picture on the left of the master cylinder.

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2 - Master Cylinder when Brake is Applied

View attachment 229795

In Diagram 2, the piston has been pushed to the left. Fluid under pressure (pink) is forced down the brake line to the pistons in the brake calliper. Note that the primary seal (blue) has moved past the compensating port, preventing any fluid from returning to the reservoir. The fluid under pressure is responsible for pushing out the pistons in the brake calliper which push the brake pads onto the disc rotors. Once the brake pads are in contact with the disc rotors it takes only a small movement in the master cylinder (brake lever) to exert a force sufficient to bring the bike to a halt.

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3 - Releasing the Brake Lever

View attachment 229796

When the brake lever is released, the piston and plunger are returned to the initial 'relaxed' position primarily as a result of the spring pressure in the master cylinder, but aided by the flexible hoses and the calliper piston seals returning to their normal state.

Each time the brakes are applied, the pads wear down a tiny fraction, and the calliper pistons are pushed out further than they were before the brakes were applied. When the brakes are released, the spring returns the master cylinder piston to its relaxed state and a negative pressure (vacuum) is created in the brake lines. Fluid is able to flow past the lips of the primary seal to allow for the fact that the pads have worn down a little.

The diagram shows the piston towards the end of its travel back to its 'relaxed' state, and the red arrow indicates the flow of extra fluid from the yellow reservoir fluid and into the brake lines (green fluid).


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4 - Dealing with Pressure Build-up

View attachment 229797

When the brake is not being applied, and the system is in a relaxed state, it is important that fluid is able to 'flow' to and from the reservoir as required. This is can happen when:-

• A slight warps in the disc rotors or general chattering of the brakes over bumpy surfaces pushes the pistons in slightly.
• An increases in temperature causes the fluid to expand.
• The bike cools down in the garage overnight and the fluid contracts. The relief port allows fluid to flow into the brake lines rather than suck the calliper pistons back.
• The brake pads are replaced and the calliper pistons are pushed in. The displaced fluid returns to the reservoir through the tiny relief port. No damage can result in pushing in the calliper pistons in this way if the system has been flushed with new fluid and the exposed calliper pistons are clean. Otherwise it is better to expel the old fluid from the calliper bores via the bleed valve.

The picture shows the pressure in the brake line being allowed to pass through the tiny compensating port into the reservoir, once the brake lever has been released. To prevent the 'fountain' shown in the diagram, a small chromed disc clips slightly above the port at the bottom of the reservoir.

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5 - Blocked Pressure Relief Port

View attachment 229798

If the piston is prevented from returning to its proper 'relaxed' state, then the Primary Seal will not expose the tiny compensating port. This could be caused by corrosion behind the piston, as shown. This part of the piston is open to the elements unless treated with silicone grease and covered with a rubber boot.
Alternatively, the compensating port itself could be blocked. Fluid that isn't replaced every year can turn into a thick gel which accumulates in the bottom of the reservoir. Or perhaps debris has fallen into the reservoir during a service.

Whatever, if the compensating port is not clear, or the piston fails to return properly to expose the compensating port, pressure builds up (red) and the brakes lock on solid. I have seen one situation recently where a master cylinder service kit was supplied incorrectly. The push rod was slightly too long and front brakes locked on solid at the first application and would not release.

----------------------------------------------------------------------

The illustrations refer to the ST1300 front master cylinder, but the principles also apply to the secondary master cylinder and the rear master cylinder, even though their relief port is connected to the fluid inlet line rather than to the reservoir itself.

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Final note. Throughout this document, I have made much more of the role of the compensating port in relieving pressure than its other functions.

In fact, the port will allow fluid to flow in either direction to add or remove fluid from the lines to compensate for the fact that the existing fluid will expand or contract as the temperature fluctuates. To do this, the master cylinder piston has to be returned to its relaxed position so that the port is not blocked by the seal. The spring in the master cylinder ensures that this happens.

I'm new to this site and just bought a 99 ST1100. No ABS. It hasn't run in 2 years. Since I've had it the only issue is my rear caliper locking up. I can break it free by loosening the bleeder. Flushed and replaced fluid but still locking up. By reading this I'm going to look more closly at the master now. Is it better to repair or replace? P.S. Still glad i chose this over the Concourse though.

 

[fnmag]

I'm new to this

I'm new to this site and just bought a 99 ST1100. No ABS. It hasn't run in 2 years. Since I've had it the only issue is my rear caliper locking up. I can break it free by loosening the bleeder. Flushed and replaced fluid but still locking up. By reading this I'm going to look more closly at the master now. Is it better to repair or replace? P.S. Still glad i chose this over the Concourse though.

Welcome to the forum Carl.
I'd go for a replacement.
It's only a few dollars more/maybe $20-$30.