rear brake sticking

One side-benefit of the goldwing recalls related to SMCs is that perhaps service departments will pay a bit more attention to the documented brake bleeding procedure. It can be a tricky one.

Update on repair:
Still don't have it back yet from the dealer. They are waiting for one last spec from tech support regarding rear wheel resistance after done applying rear brake. Hard to understand why service from tech support is so slow on a motorcycle that has been around for so many years. Oh well today might be the day!

Diamond Dave said:

Update on repair:
Still don't have it back yet from the dealer. They are waiting for one last spec from tech support regarding rear wheel resistance after done applying rear brake. Hard to understand why service from tech support is so slow on a motorcycle that has been around for so many years. Oh well today might be the day!

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They don't sell many STs in general... so, the few that have this issue are not something they are used to dealing with..

Trosty said:

Re: rear brake sticking

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No it wasn't my day. Tech support called the dealer two days ago and said "they have to review their notes".

I want to re-open this dead horse briefly. I bought a flawless 04, non abs and after only a week of riding I got the dreaded dragging rear brake. I found the post with the complex but well illustrated bleed procedure and I followed it exactly.

I didn't see much air escaping anywhere, and the front SMC seems to have its proper movement.

If I ride the bike and only use the front brakes it never does it. The minute that I use the rear brake whether by itself or in conjunction with the front it will drag, and then eventually lock the rear wheel until it sits and cools down.

I don't want to arbitrarily replace parts but.... I'm looking for some suggestions.

jzanutto said:

If I ride the bike and only use the front brakes it never does it. The minute that I use the rear brake whether by itself or in conjunction with the front it will drag, and then eventually lock the rear wheel until it sits and cools down.

I don't want to arbitrarily replace parts but.... I'm looking for some suggestions.

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Rebuild the calipers... the pistons are sticking.

paulcb said:

Rebuild the calipers... the pistons are sticking.

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+1
Also,,NEVER push those dirty pistons back in without first cleaning them very well. If the crap on the piston is forced past the outer "return" ring it will/can make the piston too tight to return to it's start point, thus dragging occurs.

a couple of questions- is there a schematic for this brake system I can look at?

it seems that the bigger center piston is directly controlled by the rear brake pedal, and the two smaller outer pistons work via the front secondary cylinder on the left wheel. Is this true?

also, if the assumption of the cruddy piston is true, why then after letting the bike sit for 30 mins does the rear brake release and function normally?

jzanutto said:

I don't want to arbitrarily replace parts but.... I'm looking for some suggestions.

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A bit more discovery is in order before you start rebuilding brake system components.

Did you try manually actuating the SMC to see if the rear brake caliper will stop the rear wheel?


Grab an 8mm wrench, a bit of tubing (fits over the caliper bleed screw) or a rag, and take the motorcycle for a spin around the block, using the rear brake to induce brake drag.
When it's obvious that the rear brake is dragging, pull off the road, turn the engine off, put the motorcycle on the center stand, and remove the left-side pannier.

This 'on the side of the road' bit is useful as it releases the brake fluid pressure in the rear brake caliper, thus allowing the motorcycle to operate, as well as aiding in the diagnosis of which (of the two) brake circuits may be at fault.

The rearward bleed screw (rear brake caliper) is a component of the SMC brake circuit, where as the forward bleed screw (rear brake caliper) is a component of the 'direct from the rear master cylinder' circuit.

Note that describing in words the hydraulic brake circuits can be awkward, and may lead to strange labels as above. Here's a link to a sketch I made when I was working on my brakes, that you may find useful.

Linked braking diagram.

https://www.st-owners.com/forums/attachment.php?attachmentid=164065&d=1431210556

SMC rebuild-

https://www.st-owners.com/forums/showthread.php?133460-ST1300-Secondary-Master-Cylinder-Rebuild-SMC

jzanutto said:

If I ride the bike and only use the front brakes it never does it. The minute that I use the rear brake whether by itself or in conjunction with the front it will drag, and then eventually lock the rear wheel until it sits and cools down.

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Since you're using the front brakes, the SMC is not the proximate cause of the rear brake lockup. Anna'sDad's check for the rear pedal check would confirm it but I agree with paulcb that the rear caliper is probably at fault for the lockup. I've heard of pads installed incorrectly causing this problem in other threads. You might try extending the pistons as far as you can and cleaning them before performing a rebuild. Use a narrow block of wood (I use several plastic door shims) to prevent the pistons from popping out. Might work, might not.

Having said the SMC is not at fault doesn't mean it's working either. Check that when you spin the rear wheel the SMC stops it when compressed manually it'll only move about 1/8 ". If not look up SMC in search for fixes.

I took the rear caliper off, took it all apart and used some mild scotchbrite to clean everything up. Lots of brake cleaner. New fluid, re assembled and rode it around.

It took a little while, but eventually the lockup came back. When I cracked the bleeder loose for the center piston (rear brake master cylinder) nothing happened. When I cracked open the bleeder for the two outer pistons (SMC system) the brake fluid came right out under pressure. Its got something to do with the linked system.

But its really odd how I can ride the bike around using only the front brake lever and I can ride for miles this way and nothing happens. But the minute I touch that rear brake pedal it locks up. I've destroyed the rear brake pads and overheated the disc. I'll replace the disc and rear pads once I get this problem eliminated.

jzanutto said:

But its really odd how I can ride the bike around using only the front brake lever and I can ride for miles this way and nothing happens. But the minute I touch that rear brake pedal it locks up.

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Yes, that's the type of discovery that leads to determining where the fault lies.

jzanutto said:

When I cracked the bleeder loose for the center piston (rear brake master cylinder) nothing happened. When I cracked open the bleeder for the two outer pistons (SMC system) the brake fluid came right out under pressure.

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And that is the proof that the SMC hydraulic circuit is causing the lock-up of the rear brake.

I imagine that when you disassemble the SMC, you will find that the piston is bound within the cylinder bore, with a fair amount of crud at the open end of the bore. When the SMC piston binds in the cylinder bore, the piston primary seal ends up blocking the compensating port, which prevents the return of brake fluid from the rear brake caliper.

When the front brakes are applied with a bound up SMC piston, the SMC is unable to 'push' brake fluid to the proportional valve, and then on to the rear brake caliper (for this discussion we can ignore the proportional valve's role in the brake circuit). So when no brake fluid is 'pushed' to the rear brake caliper, then no brake fluid can be 'trapped' in the rear brake caliper, and the rear wheel remains free to rotate.

However when the rear brake is applied with the same bound up SMC piston, brake fluid is 'pushed' into the SMC cylinder midway between the piston primary seal, and the piston secondary seal. As the rear brake continues to be applied, brake fluid is 'pushed' past the piston primary seal via the recuperation grooves, through the proportional valve, and on to the rear brake caliper, where the rear brake caliper functions correctly.

The problem arises as the rear brakes are released, the brake fluid that had been 'pushed' from the SMC out to the rear brake caliper is now 'trapped'. This is due to the SMC compensating port being blocked by the bound up piston primary seal. As the compensating port is the only return path for the brake fluid, the brake fluid is effectively 'trapped". With the brake fluid unable to return, brake fluid pressure remains steady, and the rear brake caliper does not release. With continued use of the rear brake, and the heating up of the 'trapped' brake fluid, it won't be long before the rear brake 'locks up' completely. Note that the blockage of the SMC compensating port by the piston primary seal may not be total, which is why the rear brake may release after a period of time as the brake fluid pressure slowly dissipates.

Brake fluid passing by the piston primary seal via the recuperation grooves, but failing to pass back in the opposite direction, is similar in concept to how air passes by the leather piston seal of a tire pump. As the pump handle is drawn up, air easily passes around the leather piston seal, then as the pump handle is pushed down the leather piston seal resists the air trying to pass, and instead pushes the air into the tire.

Perhaps sketch of the SMC is in order.

I think it's time to order those parts now.

A defective rear master cylinder or a bit of trash blocking the fluid return orifice on it could lock up the rear caliper then allow a slow release of fluid pressure. If it did the PCV and the SMC could both be blamed and yet be good.

yeah we'll see. I had already ordered the kit for the front SMC. One part at a time.

I think if it was the rear master cylinder then the pressure would have been in the center bleed port. But the pressure was clearly in the rear bleed port indicating that the SMC circuit was pressurized.

Anna'sDad said:

When the front brakes are applied with a bound up SMC piston, the SMC is unable to 'push' brake fluid to the proportional valve, and then on to the rear brake caliper (for this discussion we can ignore the proportional valve's role in the brake circuit). So when no brake fluid is 'pushed' to the rear brake caliper, then no brake fluid can be 'trapped' in the rear brake caliper, and the rear wheel remains free to rotate.

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Thanks for a good lesson on the SMC. I learned something valuable in maintaining my brakes.

jzanutto said:

yeah we'll see. I had already ordered the kit for the front SMC. One part at a time.

I think if it was the rear master cylinder then the pressure would have been in the center bleed port. But the pressure was clearly in the rear bleed port indicating that the SMC circuit was pressurized.

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Yes, but both circuits to the rear caliper receive fluid only from the rear master cylinder and if I understood your early post correctly you said using the front brake alone will not cause the problem.

It's easy enough to separate the fault. If you can manually manipulate the SMC and in doing so apply rear brake that immediately releases upon release of the SMC then the SMC is not causing the fault.

I had already ordered an SMC overhaul kit, and since I cant ride the bike I figured I'd take the left caliper off and open up the SMC. Snap ring, rubber boot and plunger came right off.

It's completely packed full of rust powder. and the SMC piston wont come out. I've got some cleanup to do prior to the SMC kit arriving. Hope this is the cause

jzanutto said:

I had already ordered an SMC overhaul kit, and since I cant ride the bike I figured I'd take the left caliper off and open up the SMC. Snap ring, rubber boot and plunger came right off.

It's completely packed full of rust powder. and the SMC piston wont come out. I've got some cleanup to do prior to the SMC kit arriving. Hope this is the cause

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Sounds like you should replace the complete SMC if it has that much corrosion in it.