Sounds like a Volume problem. Sure you have right bore size on Master? That's how mine acted on a truck with too small of Master. Are you aware the Hydro-vac needs bleeding also? Sometimes two bleeders on them.
These big trucks, have dual rear cylinders, and are hard to get air from one's in back, because of their position and only one bleeder on one cylinder. Sometimes you need to jack up truck or release brake line nuts to bleed those. There is an article in last months Vintage Truck magazine about bleeding the older rigs.
I believe it is better to bleed the master, the hydro next, then the CLOSEST, shortest wheel line in front, then other front, and follow rear lines back, do the one nearest frame with line on it, Then do the last farther away one. My idea is that you are following the fluid around system as you go, each time getting air farther down system. Does your hydro-vac work? Should be difference in how brakes work when running truck and when off. Sometimes, the force of hydro works the air down system easier. Look at those rear cylinders, bet that's the problem.
