I had a Case 580 K. It had a third valve in the loader circuit. I rerouted that hydraulic & used it to build a thumb. A thumb exponentialy improved the functionality of the machine.
I haven't seen your rock wall in a while Willie, it looks great! After attempting to move my rock pile around, here and there, I can imagine how handy a thumb would be. I still haven't learned the best way to move them without also tearing up the ground under them and getting a lot of dirt. The big ones I can just scoot around but they don't go very far. I've tried straps and chains but rigging methods have been poor at best. If I ever get around to building my pond out properly I'll have to learn a better way or get a thumb.
My son & I disagree on the best way to manage hydraulics. It has a hammer circuit, (one way flow) plumbed to the top of the dipper. These were available with a two way hydraulic circuit. Mine being used, it has what it has. I've looked to buying parts to convert, that looks to be very involved & expensive. I feel an electrically controlled two way valve mounted on the top of the dipper is easiest. My son believes best is a selector valve mounted beneath the tractor to divert the flow from a stabilizer circuit. I'm unconvinced. We have ordered thousands of dollars in fittings, valves & hoses. In either case, routing new hoses up the backhoe will be avoided by utilizing existing lines. A thumb doesn't work very hard retracting, no real need for a relief valve. Extending, it has relief in the supply circuit. A thumb is subject to damage when the valve isn't open. You extend the thumb, and stop it, then the bucket acting against it can increase pressure in the hydraulic cylinder. with no exit valve, the cylinder can get damaged. A "port relief" is needed to release excess pressure. Some send that oil to the filter, others send it to the other end of the hydraulic cylinder.