What's going on with my brakes?

[tim]

Riding to work this morning the front lever started feeling soft.

Then I did a long stop and it felt soft.... As i stopped i kept squeezing and the lever kept coming in and in.

CRAP

Pumped them then solid again.

I just repeated it here at work.... I got the lever all the way to the stop and could roll the bike. Let go, then they're solid... And then the sustained pressure again would stay solid.


fluid level to the top of the res.

No visible leaks.

Pads are 4 months old professionally fitted.

Fluid bled through entirely changed since then and it's been solid as it's ever been.

Thoughts? I've got the fluid to replace if need be....

[Cwidova]

maybe bleed it through again....might have an air bubble in the system somewhere.....
otherwise the only other thing i can think of is one of the seals is no good and letting the fluid through?

[Wattie]

sounds like warped disc,

you pump it up, leave it over night and see if there is pressure there in the morning. if it has lost pressure and bike hasnt moved it will bve a seal or something.

if you pump it up, then push it around then there is no pressure, then its probably a warped disc pushing the piston away from the disc and when you squeeze it, you need to take up that slack before you get action.

[tim]

Ok, Glen and I flushed the brakes but after lots of testing it became entirely clear exactly what's going on.

Pull hard on the lever, and the brakes are solid!
Pull hard and keep holding, the brakes are solid!
Pull moderately and hold, the lever comes in and in and in and in very slowly until it hits the bar, then you can ROLL THE BIKE with the lever all the way to the grip!

We suspect there's a faulty seal, probably in the brake master cylinder piston, or possibly in the caliper piston.

Our theory is that when low to moderate force is applied brake fluid is slowly able to slip past the faulty seal. When more force is applied the seal spreads under the pressure and properly seals against the cylinder wall.

There must be some mechanism for the "slipped" fluid to return back into the system, because as soon as you release the lever brake pressure returns immediately.

Off to mechanic tomorrow, don't have time to go pulling stuff apart at the moment.

It's totally mystifying to see the lever come all the way in ever so slowly, and then roll the bike with the lever all the way in, then get full pressure straight back on the discs again.

[tim]

maybe bleed it through again....might have an air bubble in the system somewhere.....
otherwise the only other thing i can think of is one of the seals is no good and letting the fluid through?

Just read this again

YOU SMART COOKIE!

haven't confirmed it yet but probably is

[tim]

Master cylinder seal kit ordered, will arrive tomorrow

[Strika]

Just for future reference, if the caliper seals are US, then fluid will leak out onto the disc surface. If you are losing pressure and not losing fluid, then the only possibilty is that fluid is leaking back past the Master Cylinder seals. The reason it only leaked at moderate brake pressure is that is the position where the M/C seal is worn the most, therefore it was first to give notice.

If it were an air bubble, firstly the rebleed done correctly, would have solved the issue and second, it would have been more of a spongy lever continuously rather than fading pressure as you experienced. Glad your on the way to getting it sorted.

[tim]

Thanks for the info Marty.

[tim]

Thought I'd just update and close this I forgot about it.

We got the master cylinder kit in and swapped it over and bled the brakes right through again - all fixed!

Thanks Rob, Glen, Marty, Wattie and Cwidova

[Troy Pearson]

Yep, order a seal kit or buy anew master cylinder for your bike!

Thats something you dont want to mess with.

Good decision I'd say!