Q: brake pedal goes to floor when starting vehicle

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I converted my 1969 dodge charger over to front disc brakes / rear drum. I also replaced the MC and power booster. the MC and lines were bled. the distribution valve was changed to a proportion valve that mounts to the same location. The pedal is hard when engine is off. the pedal goes to the floor when starting engine. thanks for your help.
My car has an automatic transmission.

It is normal for the brakes to go down about 4 inches when starting the car with foot on the brakes but not for the brakes to go to the floor. If it goes all the way to the floor then you either have rear brakes out of adjustment and brakes travel too far before hitting drums or you still have too much air in the system. If both of these are okay then you have a master cylinder that is not holding pressure and the internal compensator port is stuck or the rubber seals are torn and need to replace the master cylinder again.

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