Take a look at how that valve operates and you will see that it simply bleeds the fluid reducing the pressure at all speeds, which is not a good thing since the pressure will be lower at idle and slow speeds (parking lot maneuvers - right when we need good pressure).
I am using a real pressure reducing valve (adjustable) set at 1200 psi (for my Borgeson box) so no matter what RPMs are it will put out constant pressure. You can buy those valves at any hydraulic supplies place.
DO you have a part number or model etc for the reducing valve you bought.
Thx
Jim