Curious what happened before my mechanic comes back from the Labor Day vacation. I got stuck in traffic today, 3 hours to cover one mile, under the warm sun, running the A/C. The water temp went up and once the traffic cleared the engine was knocking loudly for a few minutes of driving and when the water temp went down the engine ran fine.

But the brakes...as soon as I changed lanes from standstill, the booster failed - hard pedal, had to jam it really hard to get any stopping power. From there on, as sketchy as it was, the booster worked intermittently, but basically I would press the brakes once, and there was no boost for the second push.

I did the standard test - turned the engine off, pumped the brake pedal till it got hard, started the engine and the pedal did not go down.

I had no chance to check out the vacuum, check valve, etc. What say you - check valve on the manifold overheated, maybe check/replace the vac hose?

Thanks!