I dunno squat about your setup.

Does your interface between the booster and the master cylinder have a "secondary freeplay adjustment"?

Some older cars have a little nut or similar that you adjust to change the length of the push rod that connects the two. An improper adjustment here can send the pedal to the floor before the brakes kick in, or make it so that the brakes don't release even with the pedal all the way up.

That's the first thing I would check..... On my old boat, the adjustment is made to the booster, where the rod comes out. There's a little nut behind the ballhead that mates with the master cylinder. I think there's even a locknut behind it. You adjust it a small distance, re connect everything, test, lather, rinse, repeat.


Although, I do think it strange that this only happens with vacuum applied. Not sure if that fits the secondary freeplay criteria. I'd check it anyway.





-tp