I have a Ford 1920 with a Ford 7108 loader. When the engine is running, the front bucket will roll down on its own very slowly. This occurs when the tractor is running or not. both bucket cylinders are not leaking from the outstide, and I cannot hear fluid seeping by the joystick control valve.
If it is only bucket curl then I would suspect the cylinders. But if you want to try a quick test, swap the lines on the valve assembly between the lift cylinders and the curl cylinders. If the curl still drifts then it's the cylinders not the valve.
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Honda 1100 Shadow Sabre 2000 (being restored)