The maximum vacuum/ pressure you can have is atmospheric, which is 14.7 psi or the equivalent vacuum, 29.9 whatever....... at sea level. Those boosters should have PLENTY of overhead for that.
In fact my recently sorrowfully bought 04 Sierra has a factory electric vacuum booster pump
I'd say either you got these from the same lame rebuilder either doing something wrong or using wrong parts, or you somehow have the wrong pushrod or adjustment, or wrong master (pushrod stroke) and are somehow bottoming something, destroying the valving
