2nd Gen Non-Engine/Transmission Brake problems

[CumminsPower98]

This is a question for a relative. They are working on a '96 with vacuum brake boost. The first time the brakes are used its not bad but if the pedal is let up and reaplied there is not enough vacuum. It takes time to build back up again. They have changed out the vacuum pump and booster using parts from a truck being parted out but the parts didn't fix it. They have checked for leaks and fixed a few small ones. The cruise and e-brake work. The pump is pulling 20 inches of vacuum. I don't know the specs. I haven't ever worked on or read much about the vacuum assist brake system in a CTD so I don't know if there are any known issues. Is there anything else? Has anyone here been down this road? I'm 75 miles from the truck so haven't looked at it myself, I'm just going by what I was told on the phone. Thanks