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Originally Posted by curtis73
Master cylinders have O-ring seals in them. As they wear against the iron or aluminum surface of the bore, they wear out. Depending on how the pressure is applied, they will "catch" and seal, or sometimes they will not seal and fall toward the floor.
Ever have one of those old-school bike pumps? As the seal wears out, sometimes it catches and pushes pressure, and sometimes you almost fall on your face because it doesn't catch pressure.
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The master cylinders I've been in all had cup seals rather than o-rings. Very similar to the bike pump Curtis is talking about above.
When they start to wear you can usually rest your foot on the pedal and have it sink slowly to the floor. But a sharp hit will seat the seals hard against the bore and make them work properly.
Often when the seals are going, there are traces of brake fluid leaking out of the cylinder and down towards the brake pedal. If brake fluid leaks inside your car it can eat carpets, matts, paint etc.