Back before we started IT racing our Z, when we were running autocross to sort our car's issues out we had a brake issue we spent months trying to solve. The car would bleed up to a good pedal, but the pedal would go to the floor when it was running and the booster was getting vacuum. Out on the course you would have a super soft pedal that would catch at the very bottom of the pedal, and when it did catch it would lock the brakes hard. Made it almost impossible to autocross. Thinking it was a hydraulic issue we changed everything hydraulic out twice, to no avail. Turned out there was a little rubber disc (called a reaction disc) in the booster that's in the path that actuates the master cylinder that had fallen out of place. Without it the brake actuation was to abrupt, and pedal was extremely soft, so it another way a booster can lead to a soft pedal. I don't believe this is the problem th OP is having, but it can be a headache to diagnose.