Hy Old GMC Gold, the hydrovac is a "slave" to the master cylinder, make sure the master cylinder is returning to its full at rest position. Check your pedal/linkage adjustment to make sure there is a small amount of clearance between the linkage and the master cylinder piston when the foot pedal is all the way up. I trust you have a return spring on the pedal/linkage. Try turning the rear wheels after they sit for 24 hours, before applying the brakes, hope that helps.
