I'm in the final stages of the resto of my early-'71, and I noticed a problem I've not seen before. When I start the car, the brake pedal drops. It really drops. If you press on it, there's no additional travel; those brakes are on. Rev it up, and they release.

It didn't do this before the restoration; they worked fine back then.

Why is the vacuum having so much affect on the booster? What changed?

Edited October 9, 20196 yr by ByStickel

Plug the line to the brake booster and see what happens. I suspect an issue with the booster

Thanks, Patcon!

I got some advice from the interwebs, and found the problem. The 'reaction disk' has fallen out of place within the booster. Worse than that, the interior of my booster is rusty from exposure to brake fluid. I'm soaking in to try to get things apart so that I can try to reinstall the disk. Having the disk fall out is a not-uncommon problem, apparently. It happens when the master cylinder is replaced.

13 hours ago, ByStickel said:

Thanks, Patcon!

I got some advice from the interwebs, and found the problem. The 'reaction disk' has fallen out of place within the booster. Worse than that, the interior of my booster is rusty from exposure to brake fluid. I'm soaking in to try to get things apart so that I can try to reinstall the disk. Having the disk fall out is a not-uncommon problem, apparently. It happens when the master cylinder is replaced.

Yes, the disc can fall out. Normally you can shake the booster enough to get it back out of the booster. But if your booster is rusty inside from brake fluid, the diaphragm inside is split or will be shortly

Yes, the old booster is toast. The rod that holds the disc is rusted in place. Luckily I did have a spare booster that's in good shape. I'm painting and swapping it in now.