Day started out fine, with a normal firm pedal. An hour later, the pedal started to feel mushy, then about 20 minutes later, went to the floor with very little resistance. No brakes at all but I stopped it with the e-brake (thank god it actually can stop a Suburban) and called a tow.

While waiting for the tow about an hour after calling, I started it and the pedal popped back up to normal and the brakes were working fine. Still took the tow and dropped it off at my mechanic, who reported no leaks and good pad and shoe wear. They couldn't replicate the problem and the brakes are working fine now.

So, any thoughts? Between the master cylinder, hydroboost and proportioning valve, which is the most likely the culprit?

Thanks guys!