Brake Problem Please Help

[k5s10]

My Dad has a 1989 Chevy 1/2 ton 4x4 that has had brake problems for a while. The pedal has alot of travel it goes almost all the way to the floor it feels like it has air in the lines,when we bleed them we get no air only fluid.Last year we replaced the master cylinder and the pedal was better but not like it should be.He has had it at the GM dealer and they can not find a problem.We have replaced all wheel cylinders and master cylinder this week and we still have the problem.Any sugestions?Thanks

 

[bronc3buster842001]

sounds like the booster is bad

 

[76zimmer]

have you tried adjusting the rear brakes out to make sure they are close to the drum when not applied?