I had a similar problem with my p/s after both box and pump were rebuilt. Long story short, it turns out that the shaft in the pump is disengaging from the pump rotor. The whole shaft and pulley work its way forewards until it disengages - then no ps, push it back in and it works. I assume some bonehead left the c-clip off the end of the shaft where it goes thru the rotor when it was being put back together.

Might be your problem, might not....