I listened to what MS told me, I got in touch with Chockostang as they specialize in stock ford power steering for early mustangs. He said that there is a relief spring that's weak or sticking because of the heat, and that the pump would be a good place to start. I ended up getting new pump, that was the problem. As you know these systems are noted for leaking, when I put the power system in 5 years ago it's been tight as a drum not so much as 1 drop under the car. I really didn't want to get into that part, if I didn't have to. I know it's only a stock set up, but between the 351w, z bar clutch linkage, 4  speed top loader, stock h pipe it gets pretty busy under there. I would never been able to do it with out MS's help, so many times he's help me out. If any ones running a stock power set up,  this may help you. Once again THANKS MS this is one great forum.