Rear Brake Binding On

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Rear Brake Binding On

I have recently rebuilt the rear brake caliper on a mates GSX600F, new seals etc. When the caliper went back on the bike and I bled up the system I found the the rear brake was binding on.

I have removed the caliper again and check that all is well, but it still binds on when I bleed it up. If I try to push the pistons back into the bore they won't budge, it's like they are locked in place. The only way they will move back in is if I open a bleed nipple.

I have today fitted a replacement master cylinder and still the same problem! I am wondering if the brake line is collapsing and so a new braided brake line is on the way but in the mean time I thought I would post here for any other suggestions??

Thanks in advance!

Sounds like either the system's over-filled or the master cylinder piston isn't releasing properly hence not letting the pistons retract. If there was debris in the brake line then I guess it could stop fluid returning to the master cylinder but it sounds a bit far fetched. Was there any debris in the old fluid or caliper when you stripped it? Another common cause is the caliper seal grooves being corroded enough that the seals don't sit properly so the pistons don't retract. If that was the case I'd expect them to move when pushed, but if you don't push exactly squarely they jam easily anyway.

I initially thought the same about the master cylinder hence getting a replacement to try, but it hasn't made a difference! I would be pretty unlucky to have two master cylinders with the same fault!

I am pretty certain that the caliper is ok. I cleaned all the seal grooves well and the pistons move perfectly freely when off the bike, it's just when bled up that I have the problem!

There didn't seem to be any crap in the caliper when I took it apart.

Like i said above, I am wondering if the hose (which is a standard rubber one) is collapsing, but opening up enough under hydraulic pressure to allow the pistons to move out??

Remove the rear brake lever, dont touch the brake line, then see if the brake is now free. Im guessing the push rod is not letting the master cylinder return all the way, ie no free play.

I have tried that, but it makes no difference. I have now replaced the brake line and still no difference!

I have come to the conclusion that it has to be something to do with the brake caliper.

I used new seals when I rebuilt the caliper. Having taken the caliper apart again and had a look at it the seals seem like a bit of a loose fit around the piston (only VERY slightly, but I am now wondering if the seals are allowing fluid in behind them and locking the piston in place)

I've ordered more new seals from another source and am hoping that this will do the trick!!

Out of interest how tight a fit should the seals be? From my recollection of doing other calipers they should be a pretty tight fit.

They should be tight enough to withstand the pressures a heavily depressed brake lever can produce.Your symptoms sound like hydraulic lock which may be that there is brake fluid getting behind the pistons preventing their return. However, in that instance, I'd expect to see evidence of fluid leak in the caliper body. You also say that the pistons return freely until they're bled up which also leads me to the same conclusion.Is the rear brake pedal adjusted properly? I know it's a long shot but it may be that it is sitting down just enough to 'activate' the brake and cause the lock. You'd find it next to impossible to return the pistons against even gentle lever pressure.Were these symptoms there before you serviced the caliper?

Rob, when you say 'brake fluid getting behind the piston' do you mean the piston seal? If so then that is what I am thinking too.

The seal doesn't seem to be tight enough that it would prevent fluid from being pushed past it under the kind of pressure that it would be put under.

In relation to fluid leaking in the caliper body, it's been difficult to tell because there has been traces of fluid on the caliper from constantly bleeding and taking apart trying to work out what on earth is going on!

I'm certain it's not the brake lever as I've removed the master cylinder from the pedal and its mountings and bled the brakes up by holding the cylinder in my hand, still the same result.

I can't say 100% whether the symptoms were there before I stripped the caliper, the old seals were pretty bad (dust seals were poking out of their recess and hanging out past the pistons! The brakes were pretty tight but nowhere near as bad as they are now!

I am pretty hopeful that the replacement seals will do the business and that for some reason the ones I was sent last time were the wrong ones! If not, I might end up torching the bloody thing!!

Further to my last, does anyone have access to what the ID and OD measurements should be for the caliper seals? Would be interesting to measure the ones that I have been sent to see if they are correct or not.

fuzzy. wrote (see)

Further to my last, does anyone have access to what the ID and OD measurements should be for the caliper seals? Would be interesting to measure the ones that I have been sent to see if they are correct or not.

Have a look on Gixxerjunkies mate.

Cheers! Ill take a look. Replacement seals arrived today so will be trying them out tomorrow! Fingers crossed!!!!

Update for those that are interested/future reference:

New seals arrived and fitted, brake caliper fitted and bled up. All is now working properly!

I'm not sure what was wrong with the old seals, nothing visually wrong with them so I can only assume that I was sent the wrong ones and the ID or OD were incorrect for the caliper.

Either way, its all sorted now and I can breathe again. The bike will be ready for BSB at Thruxton ;-0)

Nice one boss - good to hear & thanks for the update.

If I'd seen this I would have made 2 suggestions.1 Is the caliper seal in the right way, as they often have a taper, to allow the piston to retract.2 Check the resevour air hole is not blocked.

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