I have a champ model 350 hlps forklift with a four spool hydraulic control.. it has right and left

I have a champ model 350 hlps with a four spool hydraulic control...it has right and left stabilizers and tilt and vertical lift...when I activate any control it will raise the lift and will not come back down even with weight on the lift. Any ideas on what the issue is?
Check for faulty/ failing check valve in hoist cylinders