I have a 1990 d300 Cummins I have replaced everything on the front that deals with breaks except the flex lines and the booster twice but I’m still having problems with it being hard breaking and I’ve checked the booster it has plenty of vacuum. Would anyone know what to check?

 

Also I have noticed 2 vacuum lines from booster going down to what looks like 2 other vacuum things on the front where the timing cover is.

 

Do you have any pictures? Can you describe hard braking? as in the pedal is hard to push with no braking?

 

what do you mean by hard breaking?if the pedal feels the same with the truck off as it is on then the brake booster may be bad, that line going to the timing cover area is likely the line to the vacuum pump. if you got good vacuum it sounds like it is working as it should.