I wasn't sure whether to post this in Engine or here....

Has anyone ever experienced a major increase in RPM while stepping on the brakes of a power-assisted (booster) brake system?
Helping a buddy bleed his all new power brake system today and when he steps on the brakes, the engine RPM increases. The booster vacuum is attached to a manifold port on the back of the intake, directly below the carb baseplate.
Also, when I yank the distributor advance hose off while he's got his foot on the brake, the RPM's decrease. The hose it attached to the port on the lower left of the carb base as viewed from the front. It's an old Holley 4160.

I think he's got some serious vacuum leak(s) somewhere. On the vacuum nipple on the booster, it has a second smaller port that his vacuum headlight doors are attached to. We're not sure if that's where it's supposed to go, but obviously SOMETHING that uses vacuum goes there. If I remove the hose, nothing happens to the engine RPM but it has vacuum on that smaller port. I can plug it with my finger and feel it, but nothing changes.

Ideas?

You'll need to disconnect all vacuum accesorys and plug ports. Try each one, one at a time, and see what moves the rpms. Could be even the intake at this point so you might want a can of 2+2 to check for leaks. Good luck !

Leak in booster? Check the check valve also.

When he depresses the pedal, the engine should speed up a little then settle right down. If it continues to idle rough, there is a vaccum leak in the booster. Either from the pushrod side or the master cyl side.

The rear of the master cyl has a horizontal hole on the rear just below where it touches the booser, plug this and test it by applying the brakes. If you have a leak in the booster front seal, it will suck through this hole.

Excessive pedal travel due to air in the lines, can cause the engine to run rough like this but it should even out fairly quickly, not keep running rough.

Around the pushrod connected to the pedal, there is a foam "filter" it helps quiet the air rushing into the booster. this foam can rot and get sucked into the booster and perhaps into the reacton valve inside. This would cause faulty valve operation but not loss of vaccum.

When you pull the distributor hose, it's retarding the spark so the engine slows down as it should.
David

It does sound like a booster leak to me. Along with a rich mix.

OK... I was leaning towards the booster as well. It's OLD, and the only part of the braking system he didn't replace. I tipped the check valve on the booster sideways to break the seal to see if air escaped (with the car off) and didn't hear anything. That was my first clue.

We'll go over everything again and make sure there aren't any other leaks before I tell him he needs a booster. :) He's gonna LOVE to hear that. He'll probably fight me on it like he did when I told him we should bench-bleed the master before we started bleeding an entirely emtpy system. I had him pump the brakes a thousand times wondering why the rears were still dry before he finally relented and let me bleed it.
:rolleyes:

As far as the rich mix... the carb hasn't been rebuilt or tuned yet.
Thanks guys!