I'm trying to get my 30 coupe finished but dealing with "bugs." I have a dual master brake cyl. under the floor with a P&J single power booster. I'm running the vacuum line from the rear of the carb and getting vacuum. With the motor off (355 sbc), I have a good brake pedal (discs front, drums rear). When I start the engine, my pedal drops to the floor and I have no brakes. I've bled the brakes and its good. I exchanged the new booster for another and it does the same thing. I have an RV cam (no lumpy one). I "should" have no issues but can't seem to get an answer for the power assist. Anyone run into this or thoughts?