I was recruited to help a buddy work on his car (as always), he woke up the other morning and had no brakes. Although they worked fine the night before. That morning was the first cold day this fall (relatively, it's Florida), and was significantly colder than the day before. The system was topped off, and I checked for leaks at all four corners and around the cylinder, anywhere for air to get into the system. Everything was dry, and the system being full seemed to confirm that there are no leaks. When you pump the brakes a couple times, you can build up enough pressure to stop the car, but there is a hissing sound coming from the master cylinder. I'm guessing that the cold weather caused an already deteriorated seal to kick the bucket, and that it is time to replace the master cylinder. I just wanted to run it by you guys first before we threw parts at the car. Thanks in advance for any insight.
In reply to Helterskelter:
Had a hiss similar to that once and it turned out to be a rip/hole in the diaphragm of the brake booster. Was on an old GM x body.
most vacuume assist brakes will "hiss" if you pump them with the engine off. The problem is. without the booster, your brakes should be rock solid and while hard to use, should still stop the car. I doubt your problems are assist related.
sounds like booster took dump. now time to take booster to dump.
The hissing happens when the car is turned on. The pedal goes to the floor the first time you press it, then starts to feel more normal after being pumped a few times. But then it slowly loses the pressure and will go back to the floor on the next stop if enough time has elapsed. It seems to me, if it was booster related, it would be hard to push the brakes, but they would still work. Is that correct? Am I on the right track with the master cylinder?
I had a Chevette that was doing that, the master cylinder had been leaking into the booster for so long that the diaphragm ruptured in it. I had to replace the whole damn setup. 
Helterskelter wrote:
The hissing happens when the car is turned on. The pedal goes to the floor the first time you press it, then starts to feel more normal after being pumped a few times. But then it slowly loses the pressure and will go back to the floor on the next stop if enough time has elapsed. It seems to me, if it was booster related, it would be hard to push the brakes, but they would still work. Is that correct? Am I on the right track with the master cylinder?
does it hiss any time the engine is on, regardless of whether or not you're pressing the brakes?
how hard does a ruptured diaphragm make the brake pedal? that depends on how big the rupture is, ie a pinhole versus a gigantic rip.
AngryCorvair wrote:
sounds like booster took dump. now time to take booster to dump.
In Soviet Russia, booster dumps you!
If the pedal slowly descends to the floor with light pressure on it, then you have a bad seal in the M/C. You can confirm this by applying pressure to the brake pedal, which will deform the seals and seal the gap, and see if the pedal no longer drops.
Not sure about the hissing sound though, could be a combination of issues...
im going thru this same thing right now,
the booster started leaking and it took out the (old/weak) master cyl
