If the truck is original it proably has a hydrovac unit under the cab floor on the drivers side attached to the frame,you will want to look for this part and if it is there rebuild it using the guide in Tech Tips.If this is the unit you replaced then the problem could be the valve located just below the master cylinder on the firewall that the vacuum hose from intake manifold is connected to.This is a one way valve and has all kinds of effects on the hyrovac brake system.I remove mine about once a year and blow through it with 40psi compressed air as a preventive maintenace sorta thing.That valve will pop like a shotgun if it is blocked and it will flutter if it is partially blocked.There is no info on this valve in the service manual which indicates that it was probably a non serviceable item.
Hobert
