I have 1989 jeep 4.0 have replace master cylinder and I keep getting air in sys.i don’t see anyleaks
Did you bench bleed the MC?
Yes
There’re few points at which air can get drawn into the system. The MC is one, the vacuum booster is another (where the rod goes from the booster into the MC, the seal actually being part of the MC), and the calipers. When the pedal is released, that pressure drop can draw air past the “square O-ring” that seals around the piston and also draws the piston back. I’d expect that to also be leaking fluid when the brakes are applied, but since it operates sort of like a “check valve”, one direction being physically different than the other, and it’s designed to seal when under pressure in one direction specifically, it’s possible for it to draw air in when pressure is relased and not allow fluid out when the pressure is applied in the braking condition.
Beyond that the only thing I can think of is that it needed to be bled more thoroughly after the MC installation. Did you bleed it starting from the farthest caliper from the MC and working one by one toward the MC?
I did this trick once, and it won’t always work. I jacked up the left side of my truck and a neighbor pushed the brake pedal down while I loosened the lines at the master cylinder to bleed them. It got the air out. Of course I did not bench bleed the master cylinder. Wrong will sometimes work when you have no conscience.