When I started riding today I noticed I had a problem with my rear brake which to me is very strange....

when I apply the brake, it grabs fine and can lock the rear wheel no problem, however the brake lever will carry on travelling down until its reached the end of its travel, at which point I lose all braking power.

however when I release the lever and then re-apply, the brake will grab again, and like before, keep going down until there is nothing left....

It doesnt feel like there is air in the system because there is pressure to start with, plus I have bled the system and found no air bubbles coming out of either of the 2 banjo's or the bleed nipple.

any help much appreciated

All the best

Tom

Almost certainly, you have a little leak.

Fluid may leak from a seal / joint in the high pressure side of the system - i.e. between the brake master cylinder and the brake pads.

Points to check are the banjos & bleed nipple, then the seals around the pistons in the caliper. My similar problem was the tiny o-ring sealing the 2 caliper halves.

The only other fail that can cause your symptoms are fluid leaking back past the piston seal in the master cylinder.

Are you losing fluid?

Almost certainly, you have a little leak.

Fluid may leak from a seal / joint in the high pressure side of the system - i.e. between the brake master cylinder and the brake pads.

Points to check are the banjos & bleed nipple, then the seals around the pistons in the caliper. My similar problem was the tiny o-ring sealing the 2 caliper halves.

The only other fail that can cause your symptoms are fluid leaking back past the piston seal in the master cylinder.

Are you losing fluid?

more likely the second I think. Your brakes are going to last for about 10 uses maximum if you've got a leak like that I reckon. How long did it do that for? As you say it was when you started riding, and I'm presuming you still had the same when you finished, I reckon you're looking at fluid leaking back past the seal into master cylinder.

more likely the second I think. Your brakes are going to last for about 10 uses maximum if you've got a leak like that I reckon. How long did it do that for? As you say it was when you started riding, and I'm presuming you still had the same when you finished, I reckon you're looking at fluid leaking back past the seal into master cylinder.

That would make sense, I had no loss of fluid so it cant be leaking from the banjo's or nipple so it must be the master cylinder seal and its leaking straight back into the resevoir.

Ive never taken a rear master cylinder apart before, can it be done and the seal just be replaced or do I have to buy the whole thing?

Tom

That would make sense, I had no loss of fluid so it cant be leaking from the banjo's or nipple so it must be the master cylinder seal and its leaking straight back into the resevoir.

Ive never taken a rear master cylinder apart before, can it be done and the seal just be replaced or do I have to buy the whole thing?

Tom

Sounds like a good possibility the seal is malfunctioning. It's fairly easy to disassemble, but take the M/C off and try to keep it up high when you take it apart (so the line does not drain out) to make bleeding it easier later. Just keep track of the parts as they come out and you'll be fine.

the "BT280222220" Kit fits rear master cylinder number BT280222010, which I'm pretty sure you have. You can't get just the seal, but the kit will have all the internal parts you need plus the new rubber boot for the outside.

Jon

Thanks alot =]

Ive ordered the kit today, hopefully be here tomorrow and I can get it fitted.

any tips for fitting it? do I just keep all the pipes connected to it and fit the new seal etc or isnt this possible?

All the best

Tom

If it's similar to the Beta master cylinder (highly likely, they are all fairly similar) push the piston in and remove the circlip at the bottom. All the seals springs etc should come out as a stack, replace them in order, push the piston all the way back in, refit the circlip and that should do it.

Only comment is that if you have even a tiny score on the barrel of the master cylinder you'll get very similar symptoms as you initially described the cylinder kit WILL NOT solve this problem, sometimes just as easy to change the cylinder

Edited October 6, 200817 yr by Slapshot 3

Thanks alot for your time everyone, got the new seal through today, checked the old one to find a nice 1mm chunk taken out of it, managed to put the new one in without needing to bleed the system (this im quite proud of!)

also decided to put new fluid in the front brake, made more of a difference than I thought it would. Think I might change the rear brake fluid too while im at it.

Thanks again

Tom