Bear with me guys, I'm fairly new to computers and these forums. I've been messing with cars, race cars, boats, motorcyles, trucks and what-have-you for over 40 years, but this ones got me pulling my hair out. I've got a new build ('34 Plymouth sedan) with all new components. Drum brakes rear, disc brakes front. Dual-diaprahgm 8" booster w/ corvette type 1" master cylinder with dual ports mounted on the firewall. The extra ports on right side; one is plugged off and I have my brake light switch in the other one. On the rear system I have an adjustable proportioning valve and a 10 lb. residual valve both installed properly ( "in" ports toward master cylinder and "out" ports towards brakes). I have 18" vacuum at idle. Problem is: after many times bleeding ( normal two person, with speed bleeders and with vacuum pump) I have a good firm pedal, but when I start the engine, the pedal goes to the floor. I shut the engine off and again I have a good firm full pedal. I pulled the booster off and checked it on the bench with a vacuum pump and it appears to be working properly. Anybody got any ideas? Thanx

Maybe Brent Vandervort can chime in on this one. There's not much he doesn't know.

MOST CARS USE A 1.125" MASTER TO OPERATE DISC BRAKES. THE "OFF SHORE" MAIL ORDER BOOSTERS DONT HAVE MUCH POWER TO PUSH A BIG MASTER CYL. SO THEY USE A SMALL 1" PISTON TO GET PRESSURE. DISC BRAKES NEED VOLUME AND PRESSURE TO WORK. IN THIS CASE, THE FRONT CALIPER BRACKETS ARE BENT OR MISALIGNED. YOU ARE USING ALL OF THE AVAILABLE VOLUME TO BEND THE CALIPER STRAIGHT. IF THERE IS ANY DAYLIGHT BETWEEN THE PAD AND ROTOR, THAT IS THE ONE THAT IS TWEAKED. MUSTANG II CALIPERS MUST HAVE THE BLEEDER POINTING TOWARDS THE REAR TIRES TO BLEED OUT. IF THEY ARE POINTING TOWARDS THE WINDSHIELD, THEN YOU MUST SWITCH SIDES OF THE CALIPERS TO PUT THE PASSAGEWAY TO THE BLEEDER AT THE TOP OF THE CASTING. WE USE A HYDROBOOST TO GET THE PRESSURE, AND FOR CLEARANCE REASONS. THEN WE CAN EASILY PUSH THAT 1.125" MASTER, ON UP TO 1800PSI AT IDEL PUMP SPEED.

KIMDEN5, DID YO GET YOUR BRAKE PROBLEM FIXED? IF SO KEEP IN TOUCH AND LET US KNOW. MIGHT BE HELPFUL TO OTHERS WITH SAME PROBLEM...............