I have a Gravely zero-turn with Hydro Gear ZT5400 transaxles. I'm replacing the hydraulic fluid and filters for maintenance and to replace the overflow reservoirs, which had cracked. All hydraulic functions were working fine. I've drained the fluid and removed the old filters. I can't seem to turn the bypass valves from RUN to BYPASS. Is there a trick if they're stuck or turn hard? Thanks for your replies.