The relief valve may have a broken spring. Unscrew it completely and the spring and ball will fall out, or can be gotten out with a magnet. Be prepared for a little oil as well. Everything needs to be kept clean for reassembly. There is also the chance that a bit of dirt or debris got trapped in the valve. It's probably worthwhile to inspect the port with mirrors and a flashlight.

The only other thoughts I had: Check that the supply hose from the sump to the pump isn't collapsing when the pump is operated. This starves the pump for oil and could cause you to be *** in air. The other thing - check to make sure you only have one relief valve. If there's more than one, you want the one closest to the pressure supply hose.

Good luck with it.