I would be more inclined to look at the PS pump itself. Start the truck with the PS pump cap off and watch for fluid to be swirling around in the resivour. Check the fluid condition and how the brakes feel as far as assist it concerned. I just replaced the PS pump on my 98 12V that I sold and the shaft sheered off inside the PS pump. Also if your heat can change modes (floor to derfrost) the vacuum pump should be OK.
HTH
Wayne