Air in SMC Query

[Fawlty]

A month ago my mechanic did a brake fluid change on my 2002 st1300 abs. Prior to the change the brakes were fine and drag free. Recently I have noticed intermittent rear brake drag. This dragging usually starts after about half an hours riding. I notice it and stop, check the rear wheel turn on the centre stand, and the wheel is binding. I check the SMC and there is no movement. I wait about 10 minutes the wheel frees itself and the SMC operates normally. The brakes and the SMC operate normally when the bike is cold.

I have read nearly every thread in this section relating to rear brake dragging and I understand about the correct bleeding procedure, including the angling of the SMC. There is no obvious evidence of air in the system. The lever and pedal are hard and operate the brakes firmly.

I do not yet know if my mechanic followed the correct bleeding procedure.

My question for you stalwart brake surgeons here is this.

If there is air trapped in the SMC, could the heat in the system and the bike, expand the air causing the SMC to lock up?

Thanks in advance.

 

[ST Gui]

Re: Air in SMS Query

I think you're referring to the Secondary Master Cylinder aka SMC. I don't know enough to give you an accurate answer but air in the system or stuck piston in the SMC can cause the problem you're having.

I check the SM[C] and there is no movement.

How do you check it? There absolutely should be some movement— about 2-3mm forward then back to resting position. Push and pull the SMC to see if you can get it to move. It might be stuck on one position or the other. A seal may be damaged or a piston might be sticking on occasion.

Someone will be along to give you proper help and get more info from you for a better diagnosis. There may be a new SMC in your future.

 

[Fawlty]

Thanks for your reply. Yes SMC not SMS. I have amended the post thanks. I didn't make myself very clear. When the bike is cold, everything is normal and I can move the SMC as you describe. It is only when the brakes drag that the SMC will not move, once the brakes have freed (on cooling down) the SMC operates normally and will stop the rear wheel turning when I move it towards the fork. Also, when I release the SMC the rear wheel moves freely again. I cannot do that until the brakes cool. Hope that makes a bit more sense.

Incidentally, until joining this forum I had no idea of the complexities of the LBS on this machine. I am not very handy with the wrenches, although I am competent at oil and filter changes, i prefer to have the brakes done by a mechanic.

Having researched here I am now conversant with the theory, and the importance of following the correct bleeding procedures.

I suspect the SMC may be shot, the only thing holding me back is the fact that it works normally until the bike is hot, plus, it was working fine all the time prior to the fluid change.

I believe, as a general rule that air in the system causes sponginess and lack of efficiency in operation. I do not have these.

 

[dduelin]

Air in the SMC would not cause this. Air bubbles do the opposite, they causes the pedal or lever to soften even more with heat.

Dirt or a piece of gunk may be blocking a compensating port. These tiny openings allow brake fluid under pressure on one side of a cylinder to return to equilibrium. There is one in the SMC and also one in the rear master cylinder. A first step I would take is another flush and bleed of the brake system.

 

[Fawlty]

Air in the SMC would not cause this. Air bubbles do the opposite, they causes the pedal or lever to soften even more with heat.

Dirt or a piece of gunk may be blocking a compensating port. These tiny openings allow brake fluid under pressure on one side of a cylinder to return to equilibrium. There is one in the SMC and also one in the rear master cylinder. A first step I would take is another flush and bleed of the brake system.

Thanks Dave. I will get the mechanic to do that. I will ensure that he follows the correct procedure, including the SMC tilt.

 

[klaproth]

I have to disagree a bit here- I had air in my system after a bleed/flush, and had the same symptoms as Fawlty....brakes worked perfect at cold temps, but as riding time increased, the temp of the rear disk rose, and the amount of drag. I re-bled, very carefully, the whole SMC/rear brake circuit, and things went back to normal. It took a while!

NOTE: I did observe a bit of air exiting the bleeders as I bled them.

 

[Fawlty]

I have to disagree a bit here- I had air in my system after a bleed/flush, and had the same symptoms as Fawlty....brakes worked perfect at cold temps, but as riding time increased, the temp of the rear disk rose, and the amount of drag. I re-bled, very carefully, the whole SMC/rear brake circuit, and things went back to normal. It took a while!

NOTE: I did observe a bit of air exiting the bleeders as I bled them.

Noted, thanks. Helps to find someone with the same symptoms. Sounds like a good flush out is needed. Will keep you posted.

 

[jfheath]

The symptoms you describe are not entirely consistent with the conclusions being drawn.

It may well be that there is air in the system - the SMC will move only 2mm when it is new and bled properly. I know, I've just replaced mine and the pistons seals in all 3 calipers and thoroughly bled it. It is better than it was when new. Much more than 2mm and there is probably air in the system still.

But the SMC being unable to move after being ridden is an issue and that isn't caused by air in the system.

What position is it stuck in ? released, or applied. How do you know - it can be stuck in either position and both will feel the same - stuck ! Given that you have free movement when the 'brakes have cooled down' and that you can apply the rear brakes using the SMC and they release when you release the SMC, it suggests that the SMC movement is OK. (But it doesn't prove it to be OK).

But go and press hard on the rear brake pedal, and then try it again. Is the wheel free to move ? Do you get the same movement out of the SMC ? or More ? Or less ? Is the rear wheel then free after you release the SMC?

(If you can get a friend to observe the behavior of the SMC when you return from a ride, and get them to watch what happens when you press on the brake pedal, they may observe a movement of the SMC away from the fork leg. That would be an interesting and revealing observation.

Also, you may observe that with the rear brake applied, you cannot move the SMC. Get a friend or put a weight on the pedal to apply the rear brake, and try to move the SMC. The brake pedal will push the SMC way from the fork leg into its relaxed position. After pressing the SMC by hand, any movement at the SMC by pressing the brake pedal would suggest that the SMC is not returning fully to its relaxed state by itself. This may be due to a problem with the SMC piston not clearing the SMC compensation port, or it may be that the port is clear but fluid is able to escape for some other reason. The next items up the line (apart from the delay valve) is the rear brake master cylinder. It may be that the rear brake is applied, blocking its compensation port, or perhaps the returning fluid cannot escape into the reservoir - maybe because the reservoir is too full.

Check the level of fluid in the reservoir. It is not unknown for fluid to be topped up during the lifetime of the brake pads, and then when new pads are fitted, pushing in the pistons returns more fluid to the reservoir than the reservoir can accommodate.

I have a couple of articles on the forum. A few 'gotchas' which can affect the braking system irrespective of any bleeding issues, and some observations of how the air bubbles can hide when bleeding.
If you haven't read them, they are worth it. Not just my opinion - there are a couple of dealers around here that I know have used my notes !!

https://www.st-owners.com/forums/showthread.php?135125-ST1300-Brakes-Avoiding-the-Pitfalls
https://www.st-owners.com/forums/sh...0-Getting-all-the-air-out-of-the-brake-system

But if you can do those simple tests with your brake pedal and SMC and let us know what you find, it might help to identify why the SMC seems to be stuck and then becomes free.

Of course, it might be the start of the dreaded SMC problem where crud at the 'open' end of the cylinder is preventing the return of the piston and the expansion of the hot brake fluid in the rear line is causing the locking - releasing as the fluid cools. But you might expect in that case that when you apply the SMC by hand, then release it that this would also not allow the brake to release as you describe that it does.

Your bike is 16 years old. A faulty SMC might well be the issue. The design was changed in 2008 and the new design will not fit the older bikes without changing the calipers as well. The 10 year window for the supply of parts for 2002 to 2007 bikes has just passed. You might want to get an SMC anyway, while you are still able.

 

[Fawlty]

Thanks for this extensive and helpful reply John. I have already read and saved both your pdf articles. I am taking the bike to the mechanic on Monday. He is going to examine the SMC and depending on the result of that examination I will decide what to do next, taking into consideration your observations. I have a lot of faith in my mechanic, who is Spanish (and speaks no English), but I don't think he knows the intricacies of the ST1300, as they are not common over here. I am a competent Spanish speaker, but find technical translations challenging. However, armed with the information gained here, I feel we should be able to resolve things. I will be reporting back in due course.

 

[technotony007]

Hi Fawlty,
i would prepare to buy a new SMC , these are about £180 from Honda. I had similar issues and not knowing the history or how well the bike has been maintained,I bought one & a new rear master cylinder. It will save a lot of further expense with garage bills in the long run.

Then you know these parts are good in the system.

tony

 

[Fawlty]

Also, you may observe that with the rear brake applied, you cannot move the SMC. Get a friend or put a weight on the pedal to apply the rear brake, and try to move the SMC. The brake pedal will push the SMC way from the fork leg into its relaxed position. After pressing the SMC by hand, any movement at the SMC by pressing the brake pedal would suggest that the SMC is not returning fully to its relaxed state by itself. This may be due to a problem with the SMC piston not clearing the SMC compensation port, or it may be that the port is clear but fluid is able to escape for some other reason. The next items up the line (apart from the delay valve) is the rear brake master cylinder. It may be that the rear brake is applied, blocking its compensation port, or perhaps the returning fluid cannot escape into the reservoir - maybe because the reservoir is too full.

Check the level of fluid in the reservoir. It is not unknown for fluid to be topped up during the lifetime of the brake pads, and then when new pads are fitted, pushing in the pistons returns more fluid to the reservoir than the reservoir can accommodate..

Sorry to restart this thread after so long. I thought this problem was resolved but evidently not. The brakes were bled again yesterday and the rear brake pedal was firm. Got up this morning to find the pedal spongy. Left bike for 2 hours and when I returned the pedal was firm again (during that two hours the bike was left outside in temperatures of around 27c). Just been out for a ride and had the rear wheel binding. Stopped and put bike on centre stand and sure enough it was binding but not locked. Pressed hard on rear brake pedal and rear wheel freed up. Rode home without further issue.

I have done the test you suggested, and with the rear brake operated the SMC cannot be moved, when the rear brake is released the SMC returns to its released position.

I have also checked the operation of the SMC by operating it and it activates the rear brake. The rear brake releases ok when the SMC is released.

I cannot understand why the pedal went from firm to spongy and then back to firm again without the bike being ridden. Any ideas?

With regards to the SMC. I have a new one on order. Is it probable that the needle bearings could cause these symptoms? (I am referring to the binding)

Any help or advise appreciated as always.

 

[jfheath]

Stopped and put bike on centre stand and sure enough it was binding but not locked. Pressed hard on rear brake pedal and rear wheel freed up. Rode home without further issue.

That to me suggests that the SMC is not returning to its full extent by itself. Something is stopping it. When you press hard on the brake pedal, the fluid from the rear pedal enters the SMC in between the two seals - the primary and the secondary. Normally the SMC plunger would be in its fully extended state, and the fluid from the brake pedal would pass by the lips of the primary seal to activate the rear brake pedal. (This when the bike is stationary).
The fact that the rear brake releases after pressing hard on the brake pedal suggests that the fluid is pushing against the secondary seal of the SMC (closest to the plunger) and is able to push the SMC back a little further than it was able to go by itself. This will move the primary seal past the compensation port in the SMC and allow the pressure that the SMC has been exerting on the rear outer pistons to escape.

There is another situation that will cause this to happen, and that is if the SMC cylinder has been distorted slightly. Over time, when the SMC is applied, the seal isn't perfect (round seal in an oval bore) and under pressure fluid will escape past the primary seal instead of applying full force to the rear brake. This results in too much fluid being behind the primary seal and it takes time to be relieved either past the seal or via the Rear Brake Master Cylinder's compensation port. It can be a slow process. But the feel is that you keep getting a soft brake pedal. Pump it up and it remains firm for a bit, and gradually becomes soft again.

You could confirm whether or not this was the case by feeling SMC when you apply the brake pedal (a clever trick if you can do it - get some help). But you may only get one chance to feel this effect.

Is it probable that the needle bearings could cause these symptoms? (I am referring to the binding)

Yes, that is possible. But you would notice it when you were pumping the SMC when bleeding it - and its easy to check anyway - see how easy it is to turn the chromed bush by hand. Remove it and inspect the bearings by eye. I use my little finger in there to feel the bearings move. Do you use liquid thread lock ? (safety note - the manual says to use a new ALOC bolt). It is possible for this to flow round the back and get into the bush/bearing assembly.

I cannot understand why the pedal went from firm to spongy and then back to firm again without the bike being ridden.

Pass. But you must have tested the brakes to know that they were spongy. That in itself will have acted on the SMC piston (as described above) to push it back - after which the brakes will feel firm. After that, seepage of fluid past a seal that isn't forming properly could make it go spongy again. I'm not convinced by that last suggestion - it normally requires the SMC to be activated and then not return properly for the rear pedal to feel spongy when the SMC bore is distorted. (I say 'normally'. I've experienced the oval bore phenomenon just once on a friends bike)

If my suggestion about a distorted SMC bore is correct, the spongy rear brake is not caused by air, but by the fact that the first movement of the pedal is taken up by pushing the SMC plunger back into position. In the same way that the brake levers feel soft as you are pumping the pistons out after servicing the calipers.

With regards to the SMC. I have a new one on order.

Well that will hopefully cure everything. The existing SMC is possibly 16 years old, it stands a good chance of being faulty.

Tips - before you start to dismantle stuff and put in the new SMC

Make sure that you have the correct SMC before you start.

Gently push the pistons before while you still have fluid and pressure in the brake lines. But don't push them out too far. It would be good to eliminate the pistons as being the reason for any brake drag.​

You are looking for any rough surface - scratches or bubbles.

When you have the bleed valve open, verify that you can push the pistons back in using thumb pressure alone.

Gently ease back the rubber boot of the new SMC and apply some silicon / rubber grease to the plunger and around the hole where the plunger enters the SMC itself. It's easier with the SMC in your hand. Use a cotton bud or similar. Don't damage the boot. Practice this on the old one first to avoid damage. I pinch the boot between thumb and finger and pull it back. The rubber boot should keep out all water. This is an extra precaution.
Note the SMC inlet port and outlet port have a plastic plug fitted when new. Remove them both when you are ready, not before.

Fill the system and bleed it. When you are absolutely certain that you have got all of the air out, bleed it again and chase the two massive bubbles that will still be hiding in the 'n' bend behind the headlights - one heading from the rear reservoir to the SMC, one heading from the SMC to the PCV and rear outer pistons.
​

Let us know how you get on!

 

[Fawlty]

John, that's a great write up and many thanks. You certainly know your stuff! Just to let you know, I will not be installing the new SMC myself, a bit too advanced for me I am afraid. However, I will make sure the mechanic understands all that you have said. He doesn't speak English, so It might take take some time! I did have the SMC rebuilt a couple of months ago, but from what you have said, it may have been worn too much. I suspect it was the original. The new part is coming from the USA, so it will be a while, but I will report back when done. Cheers.

 

[Rwilson1061]

I am having the exact same problem. The smc will not return and stays compressed. A mechanic unfamiliar with st's replaced rear pads and for some reason tried to bleed the brakes. I was able to push the smc forward by hand, but after riding a mile or so the brake was hanging up and the piston on the smc is retracted. Thought the smc was corroded and stuck like others I read about. Bought a new one and installed. The old one operated smoothly when removed. Bought a vacuum bleeder, pulled all the Tupperware did the 7 step bleed and the piston on the smc once pushed stays retracted. Three hundred dollars and maybe 8 hours of work and I am back to the beginning. Any help would be greatly appreciated.

 

[jfheath]

That's a weird one - but if you reckon that the original SMC was OK - I would clean it up, dry it thoroughly and seal it away for a rainy day.

There are a number of reasons that the SMC piston may not return. The commonly understood one is corrosion in the bore preventing the free movement of the piston.
Another is the distortion of the bore, or the spring inside the bore is broken. Since the replacement SMC didn't cure your problem, then I guess this is not the cause.

So - we need to look for other possibilities. The blockage of the SMC bore is significant because it prevents the piston from clearing the tiny compensation port. When the port is cleared, pressure in the brakes line to the rear caliper outer pistons can escape. If the port is blocked, the pressure remains.

Two things that can also cause this to happen are to do with the mechanical operation of the SMC.

The lower bracket is actually a Pivot and the bolt carries a bush which rotates in a needle roller bearing in the fork leg. The lower bolt does not clamp the outer part of the SMC bracket to the fork. All it does is screw a pivot into the thread on the inside part of the SMC bracket. That lower part of the bracket is not clamped at all. If someone has (say) put shims in to take up the gap between the bracket on the fork leg and the SMC lower mount, then the SMC will be jammed, and will not be able to move freely.
The same thing can happen if an old bolt is rejuvenated with liquid thread lock, and the fluid seeps into the bearing. Also if the bearing itself is dry or rusted.

The second thing is one that isn't often considered. It's a theoretical possibility - ie not one that I have come across in practice. If the pressure in the brake fluid can escape through the compensation port in the SMC, it has then got to make its way back to the rear brake pedal reservoir. And to do that it also has to go through the tiny compensation port of the Rear brake master cylinder. If that piston isn't moving properly, that will not be possible. Also if the reservoir is overfull, the excess fluid will have nowhere to go.

Other thoughts. The length of that push rod on the SMC should not be adjusted. If it has been, then there may be an issue there - but your post indicates that the SMC is moving but will not retract.

Also - again theoretically - if the SMC is to return to its original state, then the rear caliper outer pistons have to be able to move. Just as hydraulic fluid cannot be compressed (very much), neither can it be 'stretched'. If the pistons cannot move when the pressure has been released, then the SMC piston will stay put. Perhaps the outer pistons of the rear caliper are corroded, scratched, pitted, blistered. Perhaps the seals have swollen - as may be the case if mineral grease was applied to the pistons. Maybe muck and grit have got behind the seals.

Some questions to ask - clean the rear pistons up. Can you see-saw the two outer pistons with thumb pressure ? As you push one in, the other should move out. Without undoing the bleed valve, can fluid be pushed back to the rear pedal reservoir ? (You may need help to observe this). If the rearmost bleed valve is released, can the pistons be pushed in easily ?

With the top SMC caliper mount bolt removed, can the SMC bracket be pivoted easily ?

I don't know the answer to your question I'm afraid, but if I had that problem I'd be doing something like the above, checking that each part of the system is doing what it should. Starting with the easy things, and then starting at the brakes and working up the brake lines.

Let us know how you get on - maybe someone else will have ideas too. And if you don't understand any of the above, then get back to me and I'll find some diagrams / photos and try to explain things a bit more clearly.

 

[Rwilson1061]

Mr. Heath. Thank you for your help and considerable time. The mechanic changed the rear pads when he attempted to bleed the brakes. I removed them thinking he must have installed them incorrectly causing them to drag. The pistons did move easily and one pushed out when the other was pushed in. The old pads looked good. Maybe I didn't get the air out. During steps five and six (I think) I could not get a good column of fluid. The vacuum bleeder was pulling air and fluid so fast that the rear reservoir would run low as quick as I could check it. I filled and almost drained it a couple of times for each step, but never got a hose full of fluid. Step 7 showed no air. I am going to follow your advice. Thank you very much. Rick

 

[Rwilson1061]

Not to wear you out....but a little more information. I removed the top bolt from the smc and it pivots freely. While loose, I compressed the rear outside pad with the tip of a screwdriver quite easily. Reattached top smc bolt and rode four miles without touching my front brake and little use of back brake. The rotor was hot enough to sizzle spit. Once again compressed rear piston easily with a screwdriver tip.

 

[jfheath]

Not to wear you out....but a little more information. I removed the top bolt from the smc and it pivots freely. While loose, I compressed the rear outside pad with the tip of a screwdriver quite easily. Reattached top smc bolt and rode four miles without touching my front brake and little use of back brake. The rotor was hot enough to sizzle spit. Once again compressed rear piston easily with a screwdriver tip.

Hmmm. I'm busy with something else at present, but I'll get back later this evening (UK time). Just an instant response re vacuum bleeders. I reckon that vacuum bleeders are essential for getting the required flow rate to shift bubbles along the tube (as is tapping the joints and flexing the tube). But in my experience, you never get a bubble free tube when using them. That is not to say that there is air in the lines - the vacuum bleeder sucks air past the threads of the open bleed valve and they emerge in the bleed tube - at an alarming rate. You can make it better by only opening the bleed valve just a little, and putting PTFE tape on the threads (but that means removing the valve completely and putting air back into the system), but it doesn't eliminate the problem.

Nevertheless with a strong vacuum applied and a slow flow of fluid you should have plenty of time to go round the lines tapping and flexing before the reservoir needs refilling.

The vacuum bleeder tends to produce a steady stream of bubbles in this way. Bubbles from the brake line will come in short bursts. When they I get a steady stream I assume the brake line is nearly free of air, and then I switch from the vacuum bleeder to using a non return bleed valve and go round the system a few times with that.

But non of that would cause the brakes to drag.

But Aha - he fitted new pads ? What are they - Honda or other make ? Do they have the little notch in the tab end (opposite end from the pad pin hole).
Did you have the dragging problem before the pads were changed ?

I'd be looking at that pad spring, the pads and the pad pin. Also at the amount of grease on the caliper slider pins. Just for starters.

I'll get back later.

 

[MidLife]

The vacuum bleeder tends to produce a steady stream of bubbles in this way.

Motion Pro to the rescue (Ref: Igofar)?

 

[jfheath]

Is Motion Pro similar to the non-return valve that I use - a metal one way valve that only allows fluid to flow out - no need to close the bleed valve on each pump of the pedal ?