Hi Brian,
Assumuing the hyd pump is in gear (left lever between your legs all the way forward, and little lever right in the centre either facing left or right), and the
oil level is ok, I would suggest that maybe the relief valve is stuck open, or the
hydraulic pump has been damaged from running dry. Both are mounted to the plate you refer to in your post. The suction screen for the pump is mounted to the pump also, they often clog up.
The relief valve simply unscrews off banjo fitting it passes though, and the body of it unscrews apart (after cutting the lockwire if it's still there). There is only a needle, spring, and some shims inside it, make sure you don't drop the shims.
As far as checking the pump, you could either dismantle it, or you could fill the suction side with oil and turn it over by hand whilst blocking the delivery side, it should push your finger off the port, if it leaks from the little leakage holes in the end of the pump rather than pushing your finger off, throw the pump over your shoulder!
Cheers,
Aaron