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
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
1 more answer
What do you mean air comes back through master cylinder? Was it bench bleed before it was installed?