Doppler46:
First, I am not a JD guy and only an amateur at hydraulics. But I have a couple of suggestions:
1.) Post this on the Hydraulics board, I think you will find more help there.
2.) If the earlier refernced diagram is a diagram of the boom, dipper or sick, and bucket curl parts of your valve (not sure what happened to swing) I would be mighty suspicious of the load check valves, as opposed to the relief valve. If the relief valve were bad, normally one for the entire valve body, if seems like all the valves in the body would have a problem. But the load checks are, in essence, relief valves for individual cylinders . I suspect, if you have load checks, the one for the dipper cylinder is either stuck or has a broken spring in it. Now, in terms of taking it apart, diagnosing and fixing it, this is one place the folks over on the hydraulics page could really help.
Hope this helps. Best of luck.