Hi

I stopped for gas the other day and when I started my Chevelle and went to leave, my brake pedal was soft and went to the floor. It was fine when I left my house and I had been driving about half an hour. I tried pumping the brake pedal but it ddin't work. I was able to stop the car if I really pushed hard on the brakes. By the time I got home the pedal was fine and I had brakes again. This was about a 15 minute ride. I had the master cylinder rebuilt two years ago by White Post Restorations so I don't think it is that. It happened one other time about a month ago. I had stopped and turned off the car this time also. Up until then the brakes worked fine. I have front disk/rear drum brakes. I checked the rear wheel cylinders and they didn't look like they were leaking. What could be causing this? Air in the lines? Do I just need to bleed them again?

Let me know,
Rich

Probably a stupid answer, is your vacuum OK?

If I had a problem with my vacuum, wouldn't I just lose my power assist, but still have brakes manually?

Stopping and starting the engine will have no effect on the braking system, except for the vacuum to the booster. If it occurs with the engine, it has to be vacuum/booster related.

If I had a problem with my vacuum, wouldn't I just lose my power assist, but still have brakes manually?
The pedal would barely move and it would be very hard to stop.

The pedal would barely move and it would be very hard to stop.

Or in the case I had, my booster went bad and when I'd start the engine the pedal would go to the floor by itself. The brakes still worked, but I'd have to pull the pedal up with my foot to get them to release.

I would suspect a wheel cylinder may be a problem

maybe you should check to see if any are leaking

or if a shoe is getting hung up on a ridge on the backing plate

anyway - good luck

Or in the case I had, my booster went bad and when I'd start the engine the pedal would go to the floor by itself. The brakes still worked, but I'd have to pull the pedal up with my foot to get them to release.
Good point. That is one of the things a bad booster or improper length pushrod can do, but it is not a vacuum problem. Your booster was still receiving and using the vacuum. If it is not recieving vacuum, or it is leaking vacuum, the pedal will barely move and it will be very hard to stop.