My HH ll developed the same problem after a few years of use (naturally AFTER the warranty expired), I went to a local hydraulic shop that specializes in farm/construction equipment hydraulic system repairs. They said that there was an internal leak past the piston in the cylinder and/or an internal leak past the spool valve inside the pump, either way it would take a complete removal and rebuild of either component or maybe both (many $$’s) mostly in labor.
They recommended just installing a ¼” hydraulic 90 degree ball stop valve in each BR slide cylinder hydraulic line. I had them do it, the cost was about $125. That included parts, a complete check for loose fittings, refilling the reservoir of fluid lost, bleeding the system and leak checking the complete hydraulic system including the LR slide hydraulic cylinder.
Now when going into storage for more than a few days, 1-close the BR slide, 2- then close the BR slide ball valves. For safety reasons after I close the valves I attach a piece of Velcro to cover over the BR slide operator switch as a reminder that the valves are closed. To open the BR slide, 1- open the ball valves, 2- remove the Velcro, 3- open the BR slide.
I hope this helps
May you have fair winds and following seas!
2005 Dodge, 2500, auto, 2wd. Pac brake PRXB. CB, Max Brake Brake Controller, Rhino Lining, Aero 60 gal. replacement tank.
1998 Hitchhiker ll, mod#31RLBGBW, RBW L'tl Rocker Hitch, Generac NP50 G generator