Hi Phil,
the vehicle should be fitted with a Bosch CP1 system.

There is no "valve" behind the "0" ring on the DRV.

The "valve" for want of a better name is inside the nose of the regulator.

The "0" ring is there to stop the fuel going past it. The Inlet to the "valve" is in front of the seal (there is a fine filter on the nose)

Fit a new seal and support washer to the DRV and re-fit it. Take care with the filter.

Crank the vehicle with the electrical connector removed from the DRV and record the rail pressure. Then re-connect the plug and start the vehicle.

Again record the rail pressure at idle and the duty cycle of the DRV.
Record rail pressure and duty cycle at three different RPM ranages finishing off with a good rev to full governed speed again recording the max pressure and duty cycle.

Post the results and we can help from there. HTH

Regards Mark