I've got one of those "spring chickens". Does it lug down the pump motor when you strain it? If not the pump may be worn enough so it can't make 2000 psi like it's supposed to. There doesn't seem to be a external pressure adjustment on the valve or pump. One of the two plugs under where the inlet hose goes into the hydraulic valve is the pressure relief. You're going to have to take it out and try to shim the spring some.