some of the back cylinders have a piston packing at bottom of ram ,if that leaks by internally then that may cause problem , if the back cylinders have a hose, at top of cylinder take that off and see if oil comes out , if it does than repack the piston seal at bottom of rod , good luck

I checked the return hose from control valve no fluid. the fluid is coming from a 1/4 id return line that runs down the top of the rear lift cylinder, supply line fron control valve goes to a junction block mounted on the mast, from there a line goes to the free lift cylinder and a second line fron the junction block goes to rear lift cylinder. so fluid from free lift to lower down is coming back to the junction block then down through the rear lift cylinder. i capped this 1/4 inch line and it solves the dropping issue but ofcorse the rearlift cylinder cant raise after max free lift, someone mentioned check valve where would this be? or any other ideas?

Unless a cylinder is externally leaking thru the gland, it wont cause fork drift. You can check by raising forks and then removing the return hose that goes from control valve to the reservoir. If oil is coming out as the forks are drifting, then the spool valve, or relief valve like swoop said, has most likely failed. On old trucks i've had spool valves fail due to age and infrequency of oil changes. I've also found on several occasions an entire control valve is cheaper than the single spool. just a heads up

I have the same issue after rebuilding lift cylinder both freelift and rear cylinder creeping down. Did you find your issue?

sounds like relief valve in main control valve
if it is damaged or has any dirt in it or a weak or broken spring it can cause this
worse case scenario might be a bad lift/lower spool section