Thread: Brake pedal drops by itself when engine is started

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It sounds to me like the weight of the pedal is not letting the MC get back to the position where it uncovers the port to let fluid flow back from the wheel cylinders into the MC reservoir. Since you don't want a return spring pulling it all the way back it seems that the answer is to weld on a stop tab that limits the return position that you can live with, adjust your pedal to booster rod to unload the booster at that position, then adjust your booster to MC rod to unload the MC at that position and install a return spring to pull everything back to static. You just need to be sure that you retain enough pedal stroke to fully actuate the MC w/o the booster or you could have no brakes with a loss of vacuum/booster failure.

The reason for the pedal fall on start is that the weight of the pedal is actuating the booster, and when you start the engine the vacuum sucks the diaphragm down to a partial applied spot. You need a spring, IMO.