1993 f350 460 5 spd long story short replaced rabs couldn't get air out of rear lines had a wheel cylinder seeping replaced that now I'm getting good stream of fluid and no air. Pedal is nice and firm until I start truck then it goes down about 3 inches before I get good and solid feel is that normal or what it seems to me they should be a little better than that and I have tried 3 master cylinders since Friday and bench bleed them makes no difference in pedal

 

All the basics I think I got covered rear brakes are adjusted no leaks t truck has had same pedal since I got it 4 years ago nothing I do makes a difference very tired of bleeding brakes only thing that hasn't been replaced is booster

 

It could be a bad booster check valve