Brake Master Cylinder Seal - Leak By?

[ElliottN]

Thought my brake pedal travel seemed to be long on occasions - especially if I was just easing the brake pedal on - but brake pedal/performance has remained hard and firm for all stops. Maybe I am imagining it? Checked master cylinder (original Girling single type) - no loss of fluid or leaks. Checked each caliper - no leaks. Checked booster - no external leaks (and no tell tale white smoke from brake fluid being drawn into the manifold). Checked inside footwell - no leaks and pedal and pushrod links all OK.

Give brake pedal a gentle, slow push by hand - it travels to the floor! Push it firmly and I get good firm resistance. Jump on it with foot and stand on it - rock solid and no excess travel.

I am assuming I have wear or a deformity to the piston seal, or wear to the master cylinder bore. If I push the pedal very gently, fluid leaks back past the seal and the piston moves through the bore without effecting braking. But if I give it a firm push, the initial pressure flares the seal in the cylinder, no leak by, and I get braking. OK - I have to overhaul the master cylinder, or I'm going to come to a sticky end real soon.

Anyone got any other ideas or experience with this sort of problem? I'm discounting the booster on the basis that since I'm not losing fluid it's not leaking and if it stops working, I just don't get the boost - right?

[patrics]

Hi Elliot,
This type of master cylinder doesn't tend to seal well on very slow static applications, but as you've notice a variation during normal driving then it is definitely time to do something about it.
You are right to assume it's the master cylinder so do check for bore ware but also check for debris especially around the centre valve and that nothing is stuck on the sealing face at the end of the bore.
Only use normal brake fluid to clean it.
Regards
Steve

[wojeepster]

most likely just a worn cup type seal. When the weather gets cold it will get worse (rubber get less compliant) Time to rebuild MC!