Air in SMC Query

[MidLife]

However, the fluid enters the SMC between the two seals on the SMC piston, and if the piston is not in the correct position, it will also try to push it forward (towards the forks).

Trying to follow you along the brake lines path!

Could not quite keep up on the above though, maybe I am not picturing it correctly?

-- How does the fluid entering between the two seals convince the piston to move one way rather than the other?

 

[jfheath]

Trying to follow you along the brake lines path!

Could not quite keep up on the above though, maybe I am not picturing it correctly?
-- How does the fluid entering between the two seals convince the piston to move one way rather than the other?

Ok - Thanks for trying to follow my garbled explanation anyway. Perhaps these will help - some images that I was going to turn into an animation, but never quite got round to it.

You realise that these are just my attempts to explain what I have observed from a failed SMC (turned out the bore was slightly squashed), and from applying this to try to work out where I had an air bubble after fitting a brand new SMC.

The first image is the SMC at rest.

The forks are on the left. The primary seal is the black seal on the right end of the piston. This is the one that pushes fluid to the rear outer pistons when the SMC is activated. The seal on the left is the secondary seal which prevents fluid from getting out into the big wide world and prevents water from getting in. You can see the tiny compensation port to the top right of the primary seal.


-------

The second image shows the state when the SMC has been activated by the action of the front left caliper rocking forward when the front brake pads grab the disc rotor.

The piston has moved to the right and the primary seal has moved past the compensation port - there is now no way out for the fluid and it applies pressure to the two outer pistons in the rear caliper. (And it turns pink ' cos it is under a bit of pressure !)

-----------

The third diagram below shows what happens when the rear brake pedal is applied when the bike is at rest - ie the SMC has not been activated.

Here. the rear brake pedal is applying the pressure to the fluid, and this makes the fluid turn green under pressure. The pressure will force the fluid to go anywhere that it can. It can't go to the left - the piston is still in its normal resting position. But it can seep past the primary seal which yields easily under pressure from behind and on to the rear caliper outer pistons. (Have you ever wondered how you can bleed the outer pistons by pumping the brake pedal ? It has to go through the SMC and squeeze past the primary seal.

This is normal behaviour - like a bicycle pump allows air to pass the washer on the upstroke.

If the SMC has been activated, (ie braking when the bike is moving), this cannot happen in the same way, as the SMC has already put the fluid to the rear caliper under pressure.

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

IN the final diagram, I have shown a slight blockage in the bore of the SMC to the left of the secondary seal.

The SMC piston cannot move back to its proper resting place (like in the first diagram). But if the problem is developing, then it might be able to be pushed past with a little extra force. (This is what happened with my friends slightly oval SMC bore).
The pressure from the brake pedal enters between the two seals as usual - and the fluid is just looking for a way out - it doesn't know which way is which - it is just going to push against anything that gives. The SMC hasn't returned, so the pink fluid is still under pressure, so it can't find any escape that way. It will push against the secondary seal (as it always does) and it provides that little extra oomph to help the spring to push the SMC piston back to the left.

And this is where the soft brake pedal feel can come from. On the first press, it is moving the SMC. On the second it will start to feel harder.

If when moving you apply the front brake to activate the SMC then the same diagram applies. Put your foot on the brake pedal - it should feel solid. If you release the front brake you should not normally feel a dip in the brake pedal, but if the SMC doesn't return fully by itself, then the rear brake line still has pressure in it (pink fluid) and this cannot get out through the compensation port. The dip in the pedal might indicate that the pedal is using extra fluid to push the piston to the left. At this point the primary seal can pass the compensation port, and the pressure in the rear brake line can be released.

The dip on the pedal can also mean air in the brake line as I said in the previous post. But whatever - it means that the brakes need looking at.

And yes - I know that pressing the brake pedal activates the centre piston on the front brake which then activates the SMC - which sort of puts a hole in my explanation. But what I describe, I have seen in action - so at present I am assuming that the pressure from the brake pedal is greater than the force exerted by the movement of the SMC when only the front centre piston is active on the front brake.

Anyway - its only my theories based on what I have understood from observations. So if this doesn't compute for anyone, please say so. It'll only help one of us to understand things better.

 

[Fawlty]

Thanks for this detailed reply John. I have done these tests:

You could try this when riding - clear road please. Reasonable speed. Apply the front brakes - not hard, but enough to activate the SMC. Then put your foot on the brake pedal. The brake pedal will feel firm.

The pedal does not firm up. Therefore, there must be air in the system?

Still moving and braking, keep your foot lightly on the brake pedal, and release the front brake lever. You may feel the brake pedal dip. You shouldn't, but my guess is that you will. The reason is that the pressure has come off the SMC from the application of the front brakes, and the pressure from the rear brake pedal is now able to push the SMC back - so the pedal feels soft.

The brake pedal does not dip.

You may also find that the pedal firms up if you press it a couple of times, without waiting for heat to do the same job. It will possibly stay firm until the SMC is activated, and then, unable to return under its own spring's pressure, it needs the brake pedal to push it back - with the resulting 'long' pedal.

The pedal does not firm up.

My new SMC has arrived and will be fitted next week. I will post again with the result. Once again, many thanks.

 

[MidLife]

The SMC piston cannot move back to its proper resting place (like in the first diagram). But if the problem is developing, then it might be able to be pushed past with a little extra force. (This is what happened with my friends slightly oval SMC bore).
The pressure from the brake pedal enters between the two seals as usual - and the fluid is just looking for a way out - it doesn't know which way is which - it is just going to push against anything that gives. The SMC hasn't returned, so the pink fluid is still under pressure, so it can't find any escape that way. It will push against the secondary seal (as it always does) and it provides that little extra oomph to help the spring to push the SMC piston back to the left.

Thanks for the great schematic. I think I can see it now.

-- When applying the pedal, green pressure increases until it starts exceeding pink pressure.
-- Then fluid goes over the primary seal (bicycle pump seal affect you talked about further up) and pink pressure will start going up as well until it pushes the piston back.

At one time you posted pictures (drawings?) of plastic parts somewhere in the flow path? Still wondering what these are doing. Maybe delaying the pressure into the SMC while activating the front middle with the pedal?

 

[jfheath]

Thanks for the great schematic. I think I can see it now.
-- When applying the pedal, green pressure increases until it starts exceeding pink pressure.

Well - possibly - but the only thing keeping the SMC from moving back as it should is something resisting the normal forces (eg the spring and the pink pressure). the extra (green) pressure might well be enough to overcome that resistance. and push it back anyway - no need to exceed the pink pressure. It wouldn't need to be greater than the pressure that remains in the pink area.

In the later stages - nothing is going to shift that SMC and the (pink) pressure builds up to such an extent that the caliper and disc become burning hot and the rear wheel locks up. This could be one of the early warning signs - but its not foolproof.

​

At one time you posted pictures (drawings?) of plastic parts somewhere in the flow path? Still wondering what these are doing. Maybe delaying the pressure into the SMC while activating the front middle with the pedal?

Do you mean these ? (Click to enlarge)


It is convenient to think of the compensation port as a single hole in the side of the SMC bore. That is not wrong, but it isn't the whole story.

Outside the SMC main bore and inside the inlet port casting, the two holes are joined together. Fluid can flow into the SMC through either the inlet port or through the compensation port (at a very much reduced rate, given its tiny diameter). But it can only flow out towards the rear brake master cylinder and reservoir through the tiny bore in the nylon moulding - due to the behaviour of that ting sprung one-way valve.

I always think of fluid coming in through the inlet port, and pressure relieved through the compensation port - 'cos its easier to think that way. But it looks like this device also allows pressure to be relieved from any heat build up in the SMC itself - between the two seals.

The compensation port in the SMC bore feeds into that void in the diagram labelled 'SMC cylinder side'.

The pedal does not firm up. Therefore, there must be air in the system?

Well, I would be inclined to think that. But other stuff you have said suggests that something else is going on too. I'm looking at Igofar's post #32. Now he has seen many, many more SMC issues than I have.
A friend had a similar problem to yours. He had all of the air out of the system, and his brakes were rock solid. He'd go for a ride and would get a long pedal. Soft brakes. A couple of dabs and they would firm up and stay ok for a while. It wasn't air. It was to do with the front brakes operating the SMC and it not being able to return by itself.

But you can get very similar symptoms if there is air in the line - and with a bit of experimenting when riding, you can work out where the air is likely to be.

But you can get the same symptoms of having a long pedal some of the time if you rear disk has a very slight wobble in it. You can't feel it pulsing under the pedal, but the longer you ride without touching the brakes, the more the pistons get nudged in. So that when you come to use the brakes there can be nothing there, cos the first press needs to move the pistons back out again.

So although air would be my first thought from what you describe, I'm all to aware that there are other things that could easily be wrongly attributed to air. But you have done the right thing in getting a new SMC. I had symptoms on mine and decided to get one before Honda stopped making them. It made sense to me.
​

 

[MidLife]

Thanks again for more great illustrations!

Well - possibly - but the only thing keeping the SMC from moving back as it should is something resisting the normal forces (eg the spring and the pink pressure).

As you say here, a force (resistance, sticking) is holding the piston back. Which means that another force needs to be generated to overcome the sticking.

........ the extra (green) pressure might well be enough to overcome that resistance. and push it back anyway -

That's were I am lost. For pressure to translate into a force, it needs to be applied to an area. I just can't picture the net resulting area that would generate the unsticking force? Maybe another great schematic will help on that too?

........ no need to exceed the pink pressure. It wouldn't need to be greater than the pressure that remains in the pink area.

Just mentioned exceeding the pink pressure because that's the case where I can clearly see how a recocking force is generated by the increased pressure.

First (and last) time the dealer bled my 03, the pedal was very mushy. Their answer was "they are all like that". Way back then there were plenty of STs on the floor and they invited me to check the pedals. Pedals were indeed very soft.....until I found one that was rock solid. Didn't know what a SMC was at the time, but this has puzzled me since then. Also the strange fact that even without rolling, there was a pulsation in the pedals when activating the front brake! So am very much appreciating your instructive posts on that.

 

[MidLife]

Do you mean these ? (Click to enlarge)


It is convenient to think of the compensation port as a single hole in the side of the SMC bore. That is not wrong, but it isn't the whole story.

Outside the SMC main bore and inside the inlet port casting, the two holes are joined together. Fluid can flow into the SMC through either the inlet port or through the compensation port (at a very much reduced rate, given its tiny diameter). But it can only flow out towards the rear brake master cylinder and reservoir through the tiny bore in the nylon moulding - due to the behaviour of that ting sprung one-way valve.

I always think of fluid coming in through the inlet port, and pressure relieved through the compensation port - 'cos its easier to think that way. But it looks like this device also allows pressure to be relieved from any heat build up in the SMC itself - between the two seals.

The compensation port in the SMC bore feeds into that void in the diagram labelled 'SMC cylinder side'.

​

That's exactly it. Here again, would have no idea what this is for without your schematic and explanations. Thanks again.

Looks like the check valve will help in two cases you have talked about?

1) It will open up to allow for larger faster flow when bleeding the system.

2) And also open up if larger inflow is required for pumping out the caliper pistons in the wobbly disk case you described.​

 

[jfheath]

That's were I am lost. For pressure to translate into a force, it needs to be applied to an area. I just can't picture the net resulting area that would generate the unsticking force.
​

The fluid is under pressure from the application of the brake pedal. Inside the SMC, the primary seal (on the right) gives way to allow fluid to pass as it applies pressure to the outer pistons of the rear caliper. If this has already happened, as indicated in the diagram with green and pink fluid, that part of the system is, in effect, closed off. There is no further movement and no fluid can flow.

But the pressure inside the SMC will also act on the secondary seal. This doesn't yield (because of the direction it is facing). If the SMC piston is in its proper position, then nothing can move. But if it is not in its proper position and the pressure is sufficient to overcome what is preventing it from returning, then the pressure will act on the secondary seal (left hand seal) to push the piston back to its correct position.

I suppose that which of these two happens first (the movement of the rear brake pistons or the return of the SMC piston) will depend on which one offers the least resistance.

If you have the appropriate theoretical knowldeg of this, you will have realised by now that I do not ! Please excuse any incorrect use of terminology.

​

Also the strange fact that even without rolling, there was a pulsation in the pedals when activating the front brake! So am very much appreciating your instructive posts on that.

There are a couple of other things that might affect the 'feel' at the brake lever when the pedal is pressed or released.

One is the delay valve. When the brake pedal is pressed, it activates the front left centre piston. If it is pressed harder, it activates the front right centre piston as well. The pedal may be percieved as dipping as the right centre piston is applied.

The other is the interaction of the different pressures exerted on the 3 pistons of the front calipers. This is theoretical - but for example, i would imagine that if the front brake is just lightly applied to scuff off a little speed and then the rear brake pedal is pressed hard, the action on the centre piston of the front caliper may take the braking effort away from the two outer pistons which are operated by the front lever. This may result in the lever yielding a little to take up the slack.

 

[MidLife]

If you have the appropriate theoretical knowldege of this .....

Wouldn't help much without the proper understanding of the system that I've nowhere seen explained better than through your brilliant work.

But have had hands on exposure to hydraulic devices with spool valves. And the SMC piston reminds me of such a spool, where pressure is applied between two seals.

..... the pressure will act on the secondary seal (left hand seal) to push the piston back to its correct position.

The trick here is that the pressure is not at liberty to decide to selectively acts on the secondary seal alone. It also has to act on the primary, in the exact opposite direction.

Following the spool analogy, for a spool to move in one direction or the other when the pressure is applied between the two seals, one seal diameter has to be larger than the other. Force being pressure time area, if the seal diameter is the same on both ends of the spool, the force is same in both direction and the piston (spool) won't nudge.

But once green exceeds pink, the pressure will leak around the primary, raising the pink pressure and pushing back on the piston.

If the piston is seriously stuck, as there is no benefit from the pedal pressure until green exceeds pink, it will require the serious stomping on the pedal some have reported having applied to try to get moving again.

 

[jfheath]

You are dead right, aren't you ? And your explanation:

But once green exceeds pink, the pressure will leak around the primary, raising the pink pressure and pushing back on the piston.

makes complete sense. So my observation was correct - pressing the brake pedal can cause the SMC to move forward - but my reasoning was wrong. It didn't need much extra pressure - the SMC wasn't entirely stuck - just enough to prevent it from returning by itself. It just felt like pumping a soft brake pedal.

Of course - I had ignored the fact the two seals are connected to the same piston and move together. If the SMC piston was just a single seal - it would do exactly what I was thinking.

The later generation of SMC and SMC kits do just that, in fact. The primary seal is attached to the spring, and not to the piston. That might suggest that applying the rear brake pedal would allow the piston to move back ? I wonder if this is why they modified the SMC - to allow the secondary seal to move independently to keep the bore clean ?

Larry ? David ?

Anyway MidLife - thanks for the comments and the hydraulics lesson. I should have been able to work that out - but once I get a picture in my head ……

 

[MidLife]

- - but my reasoning was wrong. ……

I wouldn't dare saying that, because the reasoning is in fact correct! In the end, it is pressure applied to the secondary that will help the piston move back.

Was only splitting hair (because of your saying this was turning academical!), trying to make the point that this could only happen after green exceeds pink, which is then the same as the primary not being there anymore and the recocking force on the secondary now being able to take over.

Doesn't make any difference for the guy who is pressing on the pedal in the case the piston only needs light convincing to move back.

But helps understand why more heavy stomping may be required if the piston is more severely stuck and more "pink" pressure has built up behind it that would need to be overcome first before any additional helping force is applied to the piston.

 

[Igofar]

I am so humbled by you guys and your descriptions and explanations :bow1::bow1::bow1:
You guys have probably forgotten more than I know.
I love reading and following your posts and comments.
Thank you so much for taking the time to put these out there for others to learn and understand from.
You guys are the best.
Igofar

 

[MidLife]

I love reading and following your posts and comments.

Igofar

Glad to be able to moderately contribute to your entertainment. Most of it only possible thanks to your own considerable contributions!

 

[MidLife]

..... SMC at rest.

From this illustration, it looks like when the bike is not rolling, pressing the pedal will also engage the two rear outers, on top of the middle one?

 

[jfheath]

From this illustration, it looks like when the bike is not rolling, pressing the pedal will also engage the two rear outers, on top of the middle one?

​

That's correct - at least, that is what I understand to be the case. The rear master cylinder has two banjo unions attached to the outlet. One goes direct to the rear centre piston. One goes to the two front centre pistons (via a delay valve in the case of the front right centre piston).
The line to the front left centre piston is attached to the SMC which feeds fluid to the two rear outer pistons. If the bike isn't moving then the fluid will pass by the SMC primary seal (and to a limited extent through the tiny compensation port) and activate the rear outer pistons as well.

If the bike is moving then the SMC will be activated due to the front left brake being applied (front centre piston). The fluid to the rear outer pistons is already under pressure, so the primary seal does not yield readily to allow more fluid through, and the compensation port will be blocked off as soon as the SMC piston moves.

I suppose if the brake pedal pressure was higher than the pressure from the activation of the SMC then the fluid would get past the primary seal - but whatever - the rear outer pistons are applied.

But then - there is a proportional control valve that limits the amount of pressure that can rear the rear outer pistons. Increase the braking force beyond a certain amount and the pressure to the rear outers is reduced.

 

[Fawlty]

Update re my problem. New SMC installed and system bled. The old SMC has signs of crud etc and the circlip was loose! The brakes are now sharp and the lever and pedal firm. However, I am still getting slight binding of the rear wheel, which seems to be affected by heat. For example, the bike is parked outside and on the centre stand. This morning at 8am I spun the rear wheel and got 2 full turns. When I tried again at midday, when the ambient temperature had risen to 30c, the wheel was binding, but not locked. I gently pressed the brake pedal and the wheel spun freely again. Could it be, for some reason, that when the brake fluid heats up it is expanding and somehow forces the wheel to bind and when I press the pedal the pressure is relieved? Clutching at straws here. Any ideas?

 

[MidLife]

........If the bike isn't moving then the fluid will pass by the SMC primary seal (and to a limited extent through the tiny compensation port) and activate the rear outer pistons as well......

Interesting. Thanks.

That may help explain why at times, when getting close to needing a bleed, a pedal that feels somewhat spongy at rest still turns out to be a bit firmer while in motion?

 

[MidLife]

Update re my problem. .......I am still getting slight binding of the rear wheel, .....I spun the rear wheel and got 2 full turns. .

Wondering if there are really reasons to be alarmed here! The White Courtesy Phone attendant would be the best reference, given the hundreds of STs he has worked on to this day vs my own sample of one. But I've never even tried to get two free turns out of mine and have never considered it as binding either.

 

[Al Gully]

I've never gotten 2 full turns out of my rear wheel. Take into consideration that the gear train is also creating some resistance. Your turning the final drive gears, the driveshaft along with on set of gears in the transmission. Give it a flick with your hand and if you don't feel resistance or binding, your good to go. Don't get to hung up the you absolutely need 2 full turns or its unrideable.

 

[Fawlty]

Thanks guys. I take your points. What I find difficult to understand is how the rear wheel can go from being free turning to binding in 4 hours, without the bike being ridden, or even touched, other than to turn the wheel.