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.