QUOTE(ss6 @ Jan 25 2004, 09:51 AM)
How does a master cylinder fail? What breaks in there?
the most common failure is the rubber piston seals quit sealing.
you've got rubber seals riding in a cast iron bore - they'll all wear eventually. with just a little bit of corrosion/rust (remember - brake fluid is hygroscopic and will suck water right out of the air) in the bore, the pits can crew up a set of seals in no time.
if yours hasn't yet - it will.
-usually- you get some advance warning of brake fluid seeping into the pedal box, where it swells the nylon bushings (if you've still got those). this is why you're advised to changes brake fluid at least every 2 yrs (every year is better, and racers-guys who run very high-temp (and more hygroscpoic) fluid change between events. and check your reservoir. a certain amount of fluid level lowering can be attributed to pad wear, but if it's -really- low, you've got a leak ...