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