I'm having an issue with my brakes after the car sat in out in the freezing cold for a week. I went to move it the other day and the pedal went straight to the floor and barely stopped by the end of the driveway. I pumped it a few times and it felt normal again but if I let it sit for a minute it goes right to the floor again and needs to be pumped up. The brakes were perfectly fine the day I parked it and I have been daily driving it for a month straight before hand with no issues.

So far I've checked the whole system for hydraulic leaks (no obvious ones found yet). I pulled the master cylinder out of the brake booster and that was dry as a bone. I pulled off the brake booster hose and tested the check valve and that was good. The car doesn't appear to be losing fluid at this point either.

Can a master cylinder be bad and not leak?

A few other odd things I've noticed are:

My boost gauge looses some vacuum while I pump the pedal up
My wideband also goes a bit lean while pumping up the pedal
After I pump up the pedal and hold it tight the pedal gradually goes to the floor right after I rev the engine and it pulls in extra vacuum as the rpm's come down. More vacuum = pedal dropping


At this point I'm just looking for some other suggestion of things to look for or at.