Intermittent brake issues...

[Jgreemo]

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!

[DmaxMaverick]

With no ABS in the mix and no leaks, that only leaves the master cylinder. There is no disconnect within the hydroboost unit. Proportioning valves can only block flow, not "absorb" it. It's likely you've been "one axle braking" for a while, and now the other has left you. Not uncommon, really.

[trbankii]

Other possibility would be moisture in the fluid - turns to steam as it gets hot, condenses back out when it cools off again. When was the last time you had it flushed?

[Jgreemo]

Thanks for the input. As the truck is back now and the brakes feel fine, my confidence in them doesn't, so I'll replace the MC and flush it this week. With the miles I plan on putting on it this summer, I'd like it to be reliable.

Really appreciate the comments.

Jon