I have a 94 Cherokee. Does not have ABS. A couple months ago, I replaced calipers, pads, rotors, wheel cylinders, shoes, drums and bled the entire system. Brakes were working perfectly up until yesterday. Had a very stiff pedal and everything was fine. While I was driving, I hit the brakes and heard a pop that sounded like it came from behind the brake pedal. I stopped for gas, and when I went to take off, I put it in drive, and it was taking off in 3rd gear. If I manually shifted in to 1, it would drop down to 1st gear. I went about 2 blocks and went into the store. When I came back out, I went to take off, and my brake pedal was going to the floor. Pumping the brakes wouldn't make it any stiffer. I still have brakes, but only when the pedal bottoms out. When the vehicle is off, it has a stiff pedal, but if you hit and hold the pedal, as soon as you start the vehicle it drops to the floor. Seems like you can hear air escaping from the little cylinder beneath the brake pedal when you pump it. I drove it home very carefully, still having to shift the transmission manually. Someone suggested that this may be a vacuum issue, so I climbed under the truck and checked to see if there were any vacuum lines loose going to the transmission and there werent. Ran the car for a few seconds, then turned it off and pulled the rubber plug off the power booster and you could hear the vacuum release, but it wasn't very strong. It has now been suggested to me that it may have blown a seal in the power booster...does this sound about right to you? I need to add that there are no fluid leaks, and that this morning, it was much cooler outside, and the transmission was working fine...i am baffled

I don't know all that much about it, but I'm sure some of the gurus will step in here soon - but definitely sounds like you pinpointed it to what I would have thought as well - at the brake booster. Figures it's the one thing you didn't swap out.

Shouldn't be the hardest thing ever, might be at worst an $80 part on rockauto - the power brake booster (A-1 CARDONE Part # 5473192) which for whatever reason has a $22 core charge (ugh), or a junkyard pull.

I would think your master cylinder should be fine, the seal(s) would be in your booster, but I'm not sure you could easily replace the inner seals?

Interested in what others have to say.

A bad brake booster gives the opposite: a high, stiff and hard(er) to depress brake pedal.

Don't I feel stupid...check the obvious first...couldn't see a fluid leak because the reservoir for the rear brakes is EMPTY. Time to fill it up and find the leak. Sorry to have wasted your time.

Dont be sorry. This is a excellent example of what you stated yourself: start simple.
It's not the explanation for your transmission issue, but def would have been the next thing that was going to be adviced regarding your brakes: check fluid levels and leaks.

Simple finds can be just as rewarding

Good to hear.