Article Brakes - Operation of the Master Cylinder

Interesting reading here. I have a question regarding bleeding the master cylinder, specifically the front brake master cylinder on the ST1300. When bleeding the banjo bolt, would you pump the brake pedal as well as the hand lever, or just the hand lever?
 
Fawlty said:
Interesting reading here. I have a question regarding bleeding the master cylinder, specifically the front brake master cylinder on the ST1300. When bleeding the banjo bolt, would you pump the brake pedal as well as the hand lever, or just the hand lever?
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just the hand lever. If doing it manually, bleed it like you would any other bleed point in the system - press, tighten, release, repeat (containing the expelled brake fluid as you go). Another benefit of the bleed banjo is that you can also use pressure and vacuum bleeding methods as well as a Speed Bleeder type fitting.
 
Thanks Beemer. I was just curious whether bleeding using the brake pedal would help expunge air from the SMC circuit.
 
Thanks for that John. I am swotting up, trying to understand the system, and I do understand all you have said. My query was specifically about bleeding the banjo bolt, which I had only recently read about in this thread. I now understand the procedure with regards to this. Once again thanks to you both.
 
Just for info; on the VFR forum an owner of a 98 model was told that he had a blocked Proportional Control Valve (PCV) and the clincher was that the part is no longer available from Honda. As I happened to have the part just lying around (I de-linked the brakes and sold my VFR earlier this year) in the interests of science I disassembled the PCV to see how serviceable it was. The answer is, very. I would expect the 1300 will use the same design so this may be of use to someone someday with a LBS 1100 or 1300.
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Remove the three screws holding the cover cap on; they were tough to remove, probably had threadlock, and needed persuasion with an impact driver. Under the cap is a large spring that holds the decompression piston in place. The spring and piston just slip right out with a couple of piston seals affixed.

On the cut valve there is a spring retainer cap held in place with a small circlip, if you push the cap in you can pick the circlip out. The cap has an o-ring seal and I needed to insert a small self-tap screw into the hole in the cap to pull that out. Underneath is a smaller spring that slips out, then the cut valve itself which can be retrieved with needle-nose pliers. This leaves the cast PCV body empty so you can blow out any residual crud. The cut valve seal appeared to have some rubber grease on it.

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TerryS said:
Just for info; .... I disassembled the PCV to see how serviceable it was. The answer is, very.
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Thank you for that. Always wondered how this valve was working and what exactly it was doing. Plenty of room for air to be trapped there! Glad the bleed procedure doesn't call for tilting it.

Any info on what position the VFR's was stuck in? What were the symptoms?
 
MidLife said:
Any info on what position the VFR's was stuck in? What were the symptoms?
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Sorry no; I offered the same info to the VFR owner and he is now waiting on a used part to arrive before pulling apart what he has. Supposedly a Honda tech told him the rear brake problem he had stemmed from a blocked PCV that could not be bled.
 
paulcb said:
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.
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Paul, what about Honda's recommendation to replace the banjo bolt washers whenever the bolts are loosened?
 
If I have done something, like rebuilding a brake caliper, that requires removing a banjo bolt, I will replace the washers. Then, I use the traditional process of bleeding the master cylinder by squeezing the lever until all bubbles disappear from the master cylinder and then bleed the brakes or slave cylinder at their bleed valves. The biggest problem is that it takes hundreds of lever squeezes to get the air bubbles to stop at the master cylinder.
 
To supplement John Heath's excellent write-up, I found the following video:-


The video may be about tandem braking systems but the basic principles are the same. Just ignore the final section about car systems.

Paul
 
jfheath said:
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)

View attachment 254508

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

View attachment 254509

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)

View attachment 254510

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

View attachment 254511

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 254512

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.
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I know I'm a tad bit late to the party regarding this thread but it's absolutely brilliant! Thank you for the information. It was a huge help.
 
jfheath said:
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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Awesome write up, many thanks for time and effort composing this. Oddly enough I have a 2005 1300 and just rebuilt entire braking system (calipers, font master, rear master & new SMC). took it for short test drive, all was fine until it started getting warm after testing brakes at low speed, then at high speed front brakes seized up. after it cooled down front outside cylinders still had pads clamped tight. I opened top bleed port and just a few drop of fluid came out, calipers immediately released. did this twice, same result. took master cylinder apart to check new seals and ports which were completely clear. caliper pistons easy to move with two outboard ones but they would see-saw and can’t push both them in together. Totallly perplexed since front brake system is basic, simple system unlike rear brakes/SMC portion of system.
Unfortunately I tossed old master cylinder piston/seals/rod so can’t compare length of rod or piston. Inspected piston/rod position by pulling off rod boot and putting handle back on. piston has a gap between it and the C-clip, estimating about 1/16” or more but very tough to see or measure. I’m thinking rod may be too long and piston seal covering return/relief port and not releasing pressure. would also account for fact front wheel drag is still there when cool but minute they start heating up, it gets worse until locking up. since fluid is expanding and pressure can’t be relieved. I’ll test brakes again and remove handle while front calipers are still clamped tight and if they release then this may be second time incorrect rod was in rebuild kit (all factory OEM parts used). Thanks again for info and will let you know if it is turns out to be rod issue you mentioned. if it’s not that then I am really at a loss to diagnose issue. also, I did check and verify center caliper piston moves freely and can easily be pushed back in once activated and partially extended by rear master cylinder.
 
If it is the push rod length, if you remove the brake lever, the brakes will free up. By that, I mean undo the locknut on the pivot bolt underneath the lever, unscrew the pivot bolt, and slide the lever out of its bracket. I can't remember if there is anything in there to drop onto the floor, but remove it slowly with your other hand undeneath in case.

Let us know.
 
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