My girl's car just started doing this last night. She hits the brakes and the pedal sinks all the way to the floor. If she pumps the pedal, it firms up for a little bit before sinking right back to the floor. It's got brake fluid in the reservoir so I doubt air contamination is the problem. I'm thinking the seals in the master cylinder have gone bad and the fluid is bleeding past them back into reservoir. But could a bad caliper or wheel cylinder have a slow leak that wouldn't show an obvious decrease in brake fluid but still cause the pedal to sink? The vehicle in question is a 96 Blazer BTW.

 

Id say the master cylinder is bypassing. If it was leaking that badly then you be able to tell instantly where the leak was.

 

I've had that problem before on a hydraulic brake system on 2 different occasions. Turned out to be a brake caliper starting to leak. It didn't take long at all to find the culprit with the fluid starting to seep out more and more.

 

Sounds like the master cylinder to me. It happened on my old truck. The reservoir would be completely full, but I'd have to pump the brakes to get it to stop and it would be almost touching the floor by the time it stopped. Had the master cylinder replaced and it took care of the problem.

 

If it were calipers leaking, then you would see brake fluid on the inside of the tires. I would also examine the brake lines to see if there are any leaks, particularly at any fittings, like at the master cylinder. It's most likely the master cylinder since you haven't seen a drop in the level in the master cylinder, which would make it obvious it's a leak. I did have that same experience with my 72 GMC sprint, and it was the master cylinder.

 

No matter what it is get it fixed at once before you have a wreck.

 

OK I got to drive the truck for myself and I'd say the pedal actually doesn't firm up at all when it's pumped. The booster just runs out of vacuum and makes the pedal harder to push as far. There is more travel in the brake pedal than there used to be but they work fine. The amount of travel in the parking brake pedal also indicates that the rear brakes (drums) are either worn out or totally misadjusted, causing too much free play in the brakes. Going to pop the rear wheels off and verify that.

Checked for leaks too and I couldn't find any.

 

If there are no leaks, and the pedal goes to the floor like you are saying, it's almost certainly the master cylinder that is broken. Happened to me just like that in a friend's car.