You may have a bad booster. There is a seal between the master cylinder and booster that is supposed to not allow fluid suction if there is a leak in the booster. It may be that the booster has a small leak but is not sucking the fluid out of the master cylinder.
Might want to slide the master cylinder forward and see if you hear the leak from around the booster pushrod seal.
No vacuum hoses cracked/leaking around that area is there?