gm p/s pump to mustang rack problem

[fredscarryall]

i have a 1950 suburban with chevy 383 stroker motor. i installed a jw rod garage mustang ifs kit and am using a factory gm power steering pump. i was told before i fire the motor with the power steering connected. i need to reduce the pressure out of the gm pump. because the gm pump puts out about 1200 lbs and the mustang racks requires 700 or 800 psi. so i bought a borgeson 899001 pressure reducing shim kit.otherwise i could blow the rack or have oversensitive steering. i put in the right amount of shims the instruction said to.my problem is i have no power steering. every once in a while it will work.but about 95% of the time it steers like a rock. i checked the serpentine belt and its not slipping,i have the new lines hooked correctly, the steering shaft doesnt seem to be binding at all and i have a new reman power steering rack. does anyone have any idea what it could be? the rack, the pump or something else?