brake master cylander on 1990 Chevrolet 1500 Pickup

when you mash brake padel air comes back through mastercylinder has new master cylinder

Asked by for the 1990 Chevrolet 1500 Pickup
2 answers 1 comment
Did you replace just the master cyl. or did you do the booster also?
and is their a bad hose or line. causing a leak
What do you mean air comes back through master cylinder? Was it bench bleed before it was installed?