1979 f250 4x4. Has a brake booster. I put new master cylinder on it and new brake lines. When I push the pedal it goes half way and then jumps to the floor. Brakes are weak. There is not much vacuum on the line to booster.

 

Well I'd ck the vacuum line connections from the intake manifold to the booster. Hoses dry rotted? And open vacuum ports?

Did you bench bleed the master cylinder properly?

Your power brake booster maybe bad too?