Could be worn vanes, could be an internal leak, could be a severely scored pump housing... you name it.
Saginaw pumps are good and strong but when they let go, they let go.
As cowboy said, installing a cooler when you put in the new pump is also a good idea.
Of course, it could be something as simple as contaminated fluid, too. Since you're running hydroboost, you may have blown a seal in the MC and gotten brake fluid in your PS pump. That'll crap things out in a hurry. If your brake fluid level is ok, try a flush and fill on the PS fluid. It couldn't hurt.