This is a 45RRTT high bay reach truck. Forks are drifting approx 5" in 5 mins. Its not leaking externally anywhere, which rules out the cylinders. Cylinders are displacement type cluster cylinders. I removed holding valve threaded into valve block and inspected visually. Seems OK. Removed return line at tank with forks raised and still no fluid. The forks are still drifting and I don;t know where the fluid i going? Any guesses??