Guys I need some opinions on a strange problem.

A friend of mine is trying to get my RC finished up, and today after getting the engine fired, when he depressed the brake pedal the engine RPM climbed, and the brake pedal stayed on the floor.

As soon as the motor was turned off the pedal "popped" back up. It was like some sort of vacuum was holding it down, but not giving any "feel" to the pedal.

The master cylinder is new, the booster is new. The brakes were recently replaced, new lines installed. and lines were bled. No leaks, etc.

With the engine turned off, there is great pedal and the brakes feel normal. When the engine is running the pedal is soft, no resistance, and stays stuck to the floor. As soon as the engine is shut off it literally pops back up, and acts normal.

Could this be a proportioning issue, or some sort of in-line valve that is getting stuck?

Is this booster related? Seems odd a brand new booster would act this way.

what ever it is I hope it is an easy and inexpensive repair. I have struggled going on 3 years now to get this thing back on the road.

I wish I had a manual brake setup for it, then I could eliminate the engine and vacuum. from the equation.

What about the inline check valve?

I haven't a clue, but sounds vacuum related to me. Mostly cause you say it works right when the motor is not running. Try unhooking the booster with the engine running, just to see what happens. New stuff can be bad sometimes.

"power brakes problem" By Hot Rod Dave. Have a look at this thread. It's right here on this page. Seems he has the same problem, only his is just started.

Split the M/C from the booster & check the clearance between the bottom of the "thimble" recess in the back of the piston to the rounded nub on the end of the pushrod, want ~1/8".

Hey Robert, that is great info. I am getting ready to install both master and booster and couldn't get a straight answer on this.
Thank You.

Hello Vinny & I missed you in Church today!. Yes an overextended rod would activate em at the slightest touch and not let them release totally & you're right some of the symptoms suggest something more (serious) going on but descrips/symptoms can be confusing & I figured an easy check (p rod clearance) would b a good start b 4 swapping any parts esp as it would need to come apart either way. I'm interested in the outcome too. The jump in RPM suggests a vac leak & there's no extra load on the eng as when the check valve opens & lets a certain amt of vac pull on it (depending on how much pedal input) the vac is doing the work on a closed system same as the vac adv canister as opposed to like an alt taking power directly from the eng

Replace the booster... A seal is failing, vacuum is getting past the control valve... I had a similar issue years ago & luckily it happened in a parking space.. If it had failed two days earlier my Challenger & I may not have survived.... Mountain roads at speed & suddenly locked brakes could be bad....