Binding rear brake (after using pedal)

pieterv

Joined
Jul 6, 2013
Messages
65
Location
UK
Bike
ST1300
Hi,

Another rear brake binding problem (on an 06 with ABS).

1. It only occurs when using the brake pedal (even when stationary), not the front brake lever.
2. Slightly opening the rear/outer (right) bleed nipple releases the rear brake. Because of this I am thinking it is not an issue with the pistons/seals .

What circuit does the outer bleed valve bleed? Is this an SMC problem. Can that be even if the bike is stationary, the brakes is not binding (after releasing bleed nipple), and the binding occurs after using pedal?

Grateful for any advice.

Pieter
 
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Thanks, I had read that one before posting. But it doesn't answer the question how the outer pistons in the rear caliper can get activated (and bind) by the pedal. The only way I can see is the SMC.
 
It is remotely possible that the return port in the rear master cylinder is blocked and preventing pressure release, this would activate the centre piston rear and two front centre pistons and thereafter via the smc the two outer rears..... so long as pressure is applied to the rear brake via residual pressure in the line, the front centres will also activate and so will the smc... of course the smc sticking due to corrosion will also cause it to apply pressure to the two outers without rear pedal pressure.....

Has this been checked or should I re-read the whole thread????

I would start with the checking the rear brake releases with the bike on the mainstand, pull the left front caliper down and back to release the smc.... and ensure that it does by trying to spin the rear wheel before applying the rear brake.... if the rear wheel spins freely, okay about a full turn, on the mainstand after releasing the rear brake then that should eliminate the centre piston and the return port in the rear master cylinder...
If you then push the front left caliper up and forward to activate the smc, while laying next to the bike, you can spin the rear wheel with your foot to see it turn and stop when you push the left front caliper up and forward..... when you let the caliper go, the wheel should turn freely again, if it doesn't, the smc is stuck or sticking.
 
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pieterv said:
Thanks, I had read that one before posting. But it doesn't answer the question how the outer pistons in the rear caliper can get activated (and bind) by the pedal. The only way I can see is the SMC.
Click to expand...

Ok, great to hear you've read that article
If you take a look at it again, you'll see your answer to your question on page two.
Look under The Front lever and The Rear pedal
As for John's statement under the rear pedal saying that the fluid bypasses the SMC, I believe fluid actually activates the SMC when the front left centre piston extends and makes contact rotor thus pivoting the SMC.
Fluid may be able to bypass if you have a improper secondary master cylinder push rod length though the pressure compensating port.

If you are pressing on the rear pedal while the bike is stopped, only the center piston should extend on the rear caliper.

Also, if you have manually pushed in your 3 rear pistons, dirt may be preventing the piston from retracting thus the dreaded brake drag.
Also you may have a dust seal that is torn or folded over when you pushed your pistons back into the bore. That could jam the pistons from retracting.
Put a light coating of brake fluid on the pistons after cleaning them and before pushing them back in. This may prevent the seal from folding over thus doubling up.
A rear caliper rebuild might be in the order.

See Mellows article on cleaning the pistons before inserting them back in the bore.
https://www.st-owners.com/forums/showthread.php?101206-ST1300-Rear-Brake-Piston-Cleaning

Brake Drag, things to check
Here's what Honda states:
? Contaminated brake pad/disc
? Misaligned wheel
? Badly worn brake pad/disc
? Warped/deformed brake disc
? Caliper not sliding properly
? Clogged/restricted fluid passage
? Sticking caliper piston
? Improper secondary master cylinder push rod length
? Faulty proportional control valve (rear brake)

Couple more things to add while your troubleshooting:

Have a basic understanding of hydraulic brake systems.
Basic understanding of the ST1300 Dual Combined Brake System-
?Linked Braking System.?
Read the Service manual. Honda manual preferred.
Read the Brake articles on this forum.

Best of Luck finding the faults
 
Thanks for all the replies.

Since the pistons retract fine when opening the bleed valve I think there is no issue with the pistons sticking due to dirt or seal problems. As far as a basic understanding of the brake system goes, that is what makes me suspect the SMC. It is only the outer pistons sticking, and they are fed from the SMC. Just weird that it gets pressure from the pedal (it happens when stationary, so not through normal SMC operation).

Took the SMC apart and it was a bit of a mess, rust under the rubber seal, piston almost stuck. So have ordered new piston kit. Hoping this will sort the proble. Will let you know when I have the parts and fitted them.

Again thanks for the feedback.
 
Al Gully said:
Ok, great to hear you've read that article
If you take a look at it again, you'll see your answer to your question on page two.
Look under The Front lever and The Rear pedal
Click to expand...

That explains the normal operation of the SMC, ie when you are riding and applying the front brake. I do understand that, but as mentioned the problem also occurs when stationary on the main stand, so no normal SMC operation.

The article also states " The Brake pedal also forces fluid past the SMC plunger and into the outer pistons in the rear caliper", that seems to me to be the function of the SMC itself (so not by forcing the liquid past the piston), but have since read that with a corroded semi-stuck SMC piston that could take place under stationary conditions (but not by design but due to the fault mentioned. So that might be the explanation.
 
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The Honda DCBS hydraulic brake system is rather more complex than it first appears. Over the past few years, I've disassembled, and sketched most of the brake components used in the system, all in an effort to understand how the system is designed, which will lead to a better understanding of the ST1300 brake system. While this is a work in progress, perhaps this information will be of assistance, as regards your earlier question:
pieterv said:
But it doesn't answer the question how the outer pistons in the rear caliper can get activated (and bind) by the pedal.
Click to expand...
Have a look at this post from this earlier thread titled ST1300 Rear Brake Problems.
 
Anna'sDad said:
Have a look at this post from this earlier thread titled ST1300 Rear Brake Problems.
Click to expand...

Yes, I came across your very helpful post (and diagram) when reading around. So, thanks for that! I think it can indeed explain my particular situation, where just applying the pedal with a stationary bike causes the rear to bind. As you pointed out, with a corroded and stuck piston, oil from the rear reservoir can get past the primary seal and activate the outer rear pistons. From the shape of the seal (slightly conical) it looks like it can work as a kind of one way valve, ie oil gets past from the pedal site, but due to the shape of the seal, not back, keeping the pressure on.

Well, I will see if rebuilding the SMC solves the problem. Will keep you informed. Thanks again for your thoughts, and making the diagram.
 
Do not try to rebuild the smc...replace the entire unit. If the piston shows signs of rust and corrosion the bore will be damaged as well.
 
pieterv said:
From the shape of the seal (slightly conical) it looks like it can work as a kind of one way valve, ie oil gets past from the pedal site, but due to the shape of the seal, not back, keeping the pressure on.
Click to expand...
I think it was John Heath (aka jfheath) that made the observation that brake fluid passing by the piston seals in one direction, and not the other, is very similar to how air passes by the seal in a hand operated tire pump.

I imagine your situation is/was that the SMC piston was not retracting far enough to reveal the compensating port, which resulted in pressurized fluid passing to the rear caliper by flowing around the SMC piston seal, then when the SMC piston failed to uncover the compensating port, the fluid pressure in the rear caliper was not released, causing the rear brake to remain engaged.

Rebuilding, or replacing the SMC is the solution.

The decision to replace/rebuild, should take into account the condition of the SMC bore, as well as your own abilities.

I have:
  • Cleaned, honed the bore, and reassembled my OEM SMC.
  • Purchased and installed a replacement SMC.
  • Subsequently rebuilt the OEM SMC, with the Honda replacement parts kit.
Unfortunately, the SMC is prone to collecting moisture, as well as allowing the moisture to seep into the bore, that's why I have a spare/rebuilt SMC sitting on the shelve ready to be put into service at a moments notice.

Sounds like you're on the right track with the SMC. Hope all goes well.
 
Anna'sDad said:
. . . Unfortunately, the SMC is prone to collecting moisture, as well as allowing the moisture to seep into the bore, . . .
Click to expand...

I thought that Honda updated the part to include a drain port on the part to allow water to run out instead of collect and then enter the internals.
 
Rebuilt the SMC. Everything fine now, no more binding of the rear brake.

Thanks again for the input.
 
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