Air in SMC Query

[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.

Ok - so it is a new SMC and it is pivoting freely. We assume that it is not the SMC that is at fault - the new one is behaving like the old one.

So something else is causing the rear brake to drag. So this is probably a problem with either the rear brakes or with the front brakes. Yes - you read that correctly.

You said in post 15 that the SMC plunger remained stuck.

You have verified that the rear outer pistons can be see-sawed.
You have not verified that the fluid can find its way back to the rear reservoir. For that to be possible, the SMC would have to be fully extended (ie pulled back).

So also to check on the rear caliper (some repeated info in here). See Avoiding the Pitfalls for photos of these issues.

Check that :

• The pad spring is not damaged and is installed the correct way round.
• The caliper slider pins are lightly silicone greased. Too much can cause an air lock which pushed the pads onto the disc rotor.
• The pad pin is not rusty, pitted or ridged
• The brake pads are not too thick (new OEM pads fit snugly with the white heat shield layer fitted). Non OEM may have thicker heat shield ?
• The rear brake pads should have a notch in the tab end of the backing plate - to match the ridge in the chromed retainer clip at the deep end of the caliper.
• You are not riding with your foot on the brake pedal.

Beware of assuming the drag is caused by the outer pistons. It may be the centre piston activated purely by the rear brake pedal. Thin feeler gauge may help to identify which is binding.

Front Calipers

If the left front brakes are binding slightly, this may be enough to activate the SMC and cause the rear brake to drag.

So - possible reasons for the front brake to drag

• Caliper pad spring is damaged or is the wrong way round (2008 on it is not possible to fit it the wrong way round but it can still be dislodged when inserting pads).
• Slider pins are too heavily lubricated - impossible to visually check without taking off the banjo unions. Instead remove the brake pads and push the caliper in and make sure it doesn't return by itself. Pull the calliper out and make sure it doesn't move in by itself.
• Pad pin check as for rear.
• You have fitted a white heat shield with new pads. There isn't enough room for a heat shield.
• You have grit between the disk back and the silvered backing plate.
• You have grit in the brake lever between the cup and the push rod
• The front wheel has been inserted incorrectly. Get this wrong and the relationship between the position of the left fork and the left disc rotor will be wrong, which may put one pad in contact with the disc rotor.

The wider spacer goes on the right hand side. The narrower spacer goes on the left SMC side.
The axle should be tightened without tightening any of the fork leg bolts. Hold with a large allen key. If that isn't possible then tighten the axle as much as possible, then clamp with the left (SMC side) fork leg pinch bolts, then tighten the axle to its full torque and then release the left pinch bolts again.
Move the left fork leg on the axle to ensure that the end of the axle is flush with the fork leg. This should give the correct 0.7mm clearance between the disc rotor surface and the caliper bracket. (You then have to bounce the forks a few times before re checking the flush axle and tightening up all 4 pinch bolts.

If the right hand pinch bolts are tightened before the axle nut, then the wheel will not be clamped properly (by the shoulder at the left hand side of the axle) and the wheel and disc rotors will be free to wander about on the axle which may cause the pads to catch on the disc (although I am sure you would feel this as soon as you started riding).
​

John

 

[Igofar]

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.

Standing by the white courtesy :call:
Email sent

 

[jfheath]

Standing by the white courtesy
Email sent

Glad you are onto this one Larry. Easier to diagnose when the dialogue isn't restricted by the day/night time delay across the pond.
I would love to know what the answer to this one is though - I was inclined to think front caliper drag but I don't know if that symptom persisted. It's all intelligent guesswork from over here.

 

[Andrew Shadow]

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 ?

Yes John, it is the same thing- Motion Pro Hydraulic Brake Bleeder.

 

[ST Gui]

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 ?

Yep that's it. It comes with some tubing that's really too light for the job and will slip off the nipple pretty easily. Replace that with some more stout stuff and it's off to the races.

 

[Rwilson1061]

Mr. Heath and Igofar, You gentlemen are very kind to take so much time to help. I had to walk away from this thing for a week. This morning I made sure the smc pivot pin and bushing were free and lubed and torqued both smc bolts to 23 lbs.. The rear master cylinder is not too full. When I compress the rear Pistons with a screw driver, I can't tell if fluid is moving back to the mc, but when I pump the rear brake backup I can see the level go down slightly. So I assume the fluid is moving. Lying down and manually pushing the smc forward, it does activate the rear brake and release as I release the smc. Movement of the smc is very slight. All the pads are new and maybe the Honda pads are causing the problem. It does seem to get less tight as I do my test drives. Mr. igofar, I don't find any emails, but would love to talk, if nothing but to say thanks for your time. My phone is on the rider assist page. Rick Willson, Cambridge MD, or I I'll put my contact on this page if you think it is safe. I can't thank you all enough.

 

[Igofar]

Houston we have contact :call:
Standing by if needed.
Hang in there Rick, we'll get you squared away in no time :yes:

 

[Al Gully]

I can't tell if fluid is moving back to the mc, but when I pump the rear brake backup I can see the level go down slightly. So I assume the fluid is moving. Lying down and manually pushing the smc forward, it does activate the rear brake and release as I release the smc. Movement of the smc is very slight.

Just adding a thought in here, maybe your rear M/C needs to be rebuilt. Was your brake fluid ever really grungy. Maybe silt and dirt is preventing it returning completely from the extended position.

 

[Fawlty]

Still waiting for my new SMC to arrive from the USA.

Today, I put the bike on the centre stand and did the test, by moving the SMC up. There was movement in the SMC, but it would not stop the rear wheel spinning, even though I used considerable force. I then spun the front wheel and applied the footbrake. The front wheel stopped spinning. I only post this here in case anyone on the future has similar symptoms. I take it this confirms that the SMC is shot?

 

[jfheath]

Still waiting for my new SMC to arrive from the USA.

Today, I put the bike on the centre stand and did the test, by moving the SMC up. There was movement in the SMC, but it would not stop the rear wheel spinning, even though I used considerable force. I then spun the front wheel and applied the footbrake. The front wheel stopped spinning. I only post this here in case anyone on the future has similar symptoms. I take it this confirms that the SMC is shot?

The rear brake pedal applies the centre piston of both front brake calipers. So you would expect the front wheel to stop. I assumed that it released OK ?

If the SMC was moving but it couldn't stop the rear wheel, it might suggest that the SMC is shot, but it is more likely to suggest that there is air between the SMC and the outer pistons of the rear brake caliper. That is a vey long circuit and it has a couple of big air traps in it - so it can be difficult to get all of the air out. How much movement. 1-2mm is all it should move.

But you have a new SMC on order - the best thing to do is switch it out. Its a 2002 model so it stands a good chance of needing to be replaced. I just mention it as you will likely get the same symptom when you put your new SMC on - until you get all of the air out. Don't forget to order some copper or aluminium washers. I use copper ones at a few pence each from the service department of the local dealer. You need 2 for each banjo union. Don't re-use the old ones - they are crush washers to ensure a proper seal.

Why did you order the SMC from the USA ? My dealer in the UK gets them from Europe.

 

[Fawlty]

Thanks again for the prompt response. The SMC does move 1-2mm approx. The front brake released fine. I suspect air as well, because the pedal is spongy, but sometimes it will firm up for a while and then go spongy again. I will get some washers. I ordered the SMC from Partzilla on price. My local dealer in Spain wanted €180. Partzilla price including postage €127.

 

[Igofar]

The fact that it will firm up for a while then go soft again, suggests the piston in the SMC is binding on corrosion.
If you can lift it up with no resistance (should feel like your pushing against pressure and or a spring) that would indicate that your piston is stuck down in the bore.
Let us know what you find when you replace it.
Good luck.

 

[Portupan]

SMC springs get bad over time.
On the dutch forum St1100-1300.eu better replacements are offered.

I have this replacement already over 2 years. Excellent results.
Enlist and contact @ome noot

Regards,
Martin

 

[Fawlty]

The fact that it will firm up for a while then go soft again, suggests the piston in the SMC is binding on corrosion.
If you can lift it up with no resistance (should feel like your pushing against pressure and or a spring) that would indicate that your piston is stuck down in the bore.
Let us know what you find when you replace it.
Good luck.

It does have some resistance, but you are probably right. I will do an update when the new one is installed. Many thanks.

 

[MidLife]

SMC springs get bad over time.
On the dutch forum St1100-1300.eu better replacements are offered.

I have this replacement already over 2 years. Excellent results.
Enlist and contact @ome noot

Regards,
Martin

Thanks. I don't have the tools and skills for the Ltimb mod, so was looking for just such a spring. But no luck joining the forum so far!

 

[Rwilson1061]

Thanks to Mr. Heath's response and several hours of Mr. Igofars time and encouragement, finally got my brakes straightened out. New smc, replaced bracket (bent rear slide), and a thorough bleed. These gentlemen are exceptionally generous with their time and knowledge. I have rarely experienced such kindness. THANK YOU.

 

[jfheath]

Thanks to Mr. Heath's response and several hours of Mr. Igofars time and encouragement, finally got my brakes straightened out. New smc, replaced bracket (bent rear slide), and a thorough bleed. These gentlemen are exceptionally generous with their time and knowledge. I have rarely experienced such kindness. THANK YOU.

Most welcome - I'm glad you got it sorted out, and thanks for letting us know of the result. It sounds like Larry is the real hero - second only to yourself for daring to delve into the unknown and get it sorted out.

A bent slider pin ? Strewth - that must have had some peculiar force applied to it.

 

[Igofar]

Most welcome on my end as well. I very much enjoyed the time spent on the phone.
You were a joy to work with on the phone, having everything prepared before our phone session.
You should be very proud of yourself for a job well done!
You accomplished more than most mechanics at the dealers were able to do.
Glad you have your bike repaired and can now enjoy riding it safely.
Igofar

 

[Fawlty]

Still waiting for SMC to be fitted. I have another question. My bike is parked outside in my front garden. I have noticed that the rear brake sponginess/firmness appears to be affected by heat. First thing in the morning the pedal is soft, but during the day as the heat builds it firms up. When it does firm up the back wheel drags a little. I have also noticed this when riding. After a few miles the pedal sometimes firms up.

When standing outside the pedal can go from soft to firm and back to soft again, as the day starts off cool, heats up (32C at the moment) and then cools again. In the morning I check the pedal by lightly pushing it once by hand a short distance. I can tell it is soft. When I check again in the late afternoon it has firmed up. This is weird behaviour indeed. Any comments guys?

 

[jfheath]

This is really an academic discussion now - but one that is worth a response. My opinions and observations only. I'd be curious to know if my observations tally with how yours is behaving.

Not that it matters a great deal - your new SMC will probably fix everything - but its nice to test my theories on another bike !


If the brake drags when heat builds up, then that is one of the signs that the primary seal on the piston inside the SMC is not able to move past the tiny compensation port in the sidewall of the piston bore. This may be due to corrosion near the exposed end preventing the piston from moving its full length; it may be due to the compensation port in the master cylinder (at the rear brake pedal is not able to allow fluid to pass through it); it may be due to the piston bore of the SMC being distorted, preventing the piston from moving freely.


If the brake pedal feels soft, this could be due to air in the system, but it could also be due to the same reasons as those above. What can happen is that the piston in the SMC cannot return fully under spring pressure - which it should be able to do. When the SMC problem is developing, and the SMC cannot move properly, it sticks. But when you press the brake pedal, the fluid feeds into the SMC and will when the bike is stationary, it will push fluid past the piston to the rear brake outer pistons.

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). So if the SMC hasn't returned, and the SMC sticking fault is just developing, then it will succeed in helping the piston to move to its correct position - but this makes the brake pedal feel soft - in the same way that brakes feel soft when the pads are not close to the brake disc rotors. You would be able to feel this movement in the SMC if you could hold the SMC and press the brake pedal at the same time.

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.

If it doesn't, you have air in your rear brake line between the pedal and the SMC. The reason is that the SMC is applying pressure to the line from the SMC to the rear outer pistons, and that line is already under pressure. If it feels soft, there must be air in the other part of the line.

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.

(I know at this point some will comment that the rear brake pedal also applies the front brake centre piston - which will also apply the SMC. I don't know the precise relationship between rear brake pressure and the resulting SMC pressure - but I do know that the rear pedal dip can present itself as a symptom of an SMC that doesn't return properly).

The expansion of fluid due to heat can easily play its part in your situation.

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.