Losing Pressure In Front Brake

[c_braddon]

when i come to ride my 05 pro i tend to have little/no front brake.

if i pump the leaver it comes back and works ok but when left to sit unused for a short amount of time it will go back to the state of less pressure and need pumping again.

this is becoming a problem as between sections of trials waiting etc i loose the brake and need to pump it again before starting the section.

any advice???

[jse]

when i come to ride my 05 pro i tend to have little/no front brake.

if i pump the leaver it comes back and works ok but when left to sit unused for a short amount of time it will go back to the state of less pressure and need pumping again.

this is becoming a problem as between sections of trials waiting etc i loose the brake and need to pump it again before starting the section.

any advice???

Provided there is (1) adequate fluid in the M/C reservoir, (2) no leakage of fluid at the fittings (loss of fluid volume) and (3) the lever pressure is good/solid after pumping it up (no compressible air in the system), my first guess is worn piston seal in the master cylinder.

If the rubber boot that the M/C plunger goes through is compromised at all, the chances that the seal is bunged are that much greater. What will happen if the boot is broken/missing is that fine grit will get into the bore and wear the seal lip. Even with normal seal wear the symptoms will be similiar.

We usually use about the first 1/3rd of the master cylinder bore in normal brake operation so that after a while the first 1/3rd of the bore will be a slightly larger diameter than the rest of the bore due to normal wear. When the seal lip is worn to the point where it looses tension with the walls of the bore, when the lever is pulled and the piston starts to travel, fluid will pass by the lip until the seal gets down into the unworn part of the bore (smaller diameter) where it will seal again and pressurize the line. With a little pressure built up in the system the seal, due to it's shape, will expand out a little and be able to start pressure at the start of lever travel. If the brake is not used for a time, the seal will relax (become slightly smaller diameter) and the process starts again.

My bet is that if you put in a M/C rebuild kit, the problem will be solved.

Jon