power steering pump going bad?

hello all i have a 1991 GMC C3500 truck 5.7l 3+3 long box the truck is not pretty at all but i like it a lot. one of the things that bother me about it is the power steering pump. on start up the belt squeals just a little, also the breaks are powered off the power steering pump as well so for the first few miles the pump belt squeals if i steer or use the brakes. I have replaced the belt and tightened it. after the truck is warmed up quiet as a mouse unless i get on the brakes. Also there is plenty of fluid.
So is the pump not keeping up with demand or is the bump going bad?
thanks