Hydraulic pressure issue

View attachment 179265 I have a John Deere 310b with hydraulic pressure issues. I put a test gauge on both low pressure and high pressure ports. When I rev the engine to 3/4 throttle the pressure will jump to 2200psi but remains at zero on the low pressure side. If I lower the rpms to about half throttle pressure drops to zero. Any help would be greatly appreciated.

Hydraulic pressure issue