Pull the pump back off and check the cam lobes that drive it. My friend has the same problem, and I had my pump start slowing down. Found the lifters had eaten into the cam lobes until the hard facing was gone, then ate further until the lift was too low. Remember pressure is a measure of resistance to flow - if there is no flow (such as gage installed and ram not) pressure will be OK. As soon as flow is required (ram hooked up and extending) the volume may be so low you get no pressure.
Cure is a new PTO shaft or weld, regrind, and harden. I fortunately had a spare shaft. Maybe someone knows the base circle and lift on the PTO cams?