Francis,

It sounds like the sequence valve, some call it the shuttle valve has failed since you have no oil in the cylinder. First, look at the top of the free lift cylinder and see if you have a bleeder screw at the top. If so, loosen the screw and raise the mast until you see oil coming out of the screw. Tighten and test mast. If you do not have a bleeder screw, remove the hydraulic block, valve on the back of the mast and check all ports. Suspect you have a stuck valve inside. Good Luck.