Could be (a)the linkage adjustment on the valve, (b)dirt or foreign matter in the oil sticking in the valve, (c)rust on the cylinder rod, (D)rusted or stuck piston rings/seal, or (d) even stuck hinges but that is remore. However I have seen that before. I'm guessing that this is an old twin arm underbody type hoist and not a later style scissors type. I would advise carefully inspecting it w/o getting under it for anything obvious and if not found then I'd drive forward and try a panic stop with the valve in the lowered position.
Try this first and if it still doesn't work then contact me privately and we'll take it from there.
