Whenever a Pre-load situation happens like this you want to check the gap between the master and the booster pin. You want a 1/32"-1/16" of freeplay between the two. Also, when converting from manual to power brakes you want to run the booster clevis though the lower hole on the pedal. Generally 1" lower then the factory manual brake position. If no other hole is there then one will need to be drilled.