We always talk about how brake fluid sucks up moisture out of the air but we don't really see the affect it has except when cars sit for a while. The lip seals in the master cylinder can stick to the bore and get flipped around the next time the pedal is pressed. Corrosion can build up in the upper area where the pistons DO normally travel. That can lead to tears and internal leakage.
One potential clue is that you mentioned the brakes work a little if you pump the pedal quickly. That sudden pressure can push the lip seals against the bore and help them seal, ... At least for that one pedal stroke. But usually you'll find the second lip seal fails a short time later, then you won't have any brakes. That second seal doesn't always fail in a few days, but when it does, that's the clue to suspect the master cylinder.
Depending on the style of cover on the reservoir, another potential clue is to remove the cover and watch for two spurts of fluid shooting straight up when a helper hits the brake pedal kind of hard. You should see that fluid shoot up as the pistons move down the bore. Once the pedal travels about an inch or two, the lip seals will have traveled past those ports and the spurting will stop. That's when the seals have moved far enough to trap that fluid and start to put pressure on it. If you are able to see where that fluid should be spurting up but it isn't happening, suspect the master cylinder. That was easier to see on older cars that had cast iron master cylinders and stamped steel covers as big as the master cylinder. Most plastic reservoirs don't allow you to see those spurts.
Even if you had a leaking rubber hose or steel line, you will still gets those spurts because those ports are the easiest place for fluid to go. Of course then the fluid level would be dropping.
Sunday, March 4th, 2012 AT 8:12 AM