'65 Mustang P/S pump pressures

I helped a neighbor install a new control valve recently, but the vehicle seems to have very little assist. I checked pump pressure at the pump - it was 50-75 psi at idle, 275-300 while attempting to turn the steering wheel. I didn't go full lock to see what that pressure was as my gauge is meant for automatic transmissions and only goes to 300 psi. I called a local rebuilder who said pressures should be in the 600+ psi range. Does anyone know what the pressures for this vehicle should be? It has a 289 w/automatic if that makes any difference. Thanks for any help.

Dave

The Shop Manual for 62 Ford Fairlane 500 that I have shows the pressure should be 750--900 psi. I also checked my Glen's 1965 Auto Repair Manual it shows the pressure should be 800--1000 psi. Also my Ford Manual says to allow at least two minuets for the fluid to warm up. If pressure using a testing tool with the shutoff valve fully closed if the pressure is still less than 750 psi the pump is causing the problem. If the pressure increases to 750--900 psi the problem is either the control valve or power cylinder. The Fairlane uses a hydraulic cylinder to move the front end assembly for turning, I do not know if the Mustang uses the same system or not. I hope this helps. Dennis