we can guess at this all day long
but we need a full model number to give you any accurate information

but from what i remember on these masts the valve at the base of the freelift and left main cylinder is called a 'lowering control valve'.
It's function is to catch the load and limit the lowering speed in the case of a ruptured line. It is a safety valve. It has no bearing on how the load is held while the mast is raised.

From the diagrams i found on a 2 stage limited freelift mast the freelift (center) cylinder does have seals on the base of the piston and a flow control valve in the piston. But i did see a note that some 2 stage masts did not but there is one in the main left cylinder piston. From what i read that valve is there to allow a certain amount of fluid to pass to the top side of the piston packing to allow for some cushioning when full raised.
The troubleshooting section did speak of faulty seals in the cylinders or a possible faulty control valve. And as was suggested installing a gate valve between the control valve and the mast was the method used to determine where the leak was.

now with all that said there is still a slim possibility that this mast is a special mast not made by yale but that is rare.

Without a complete model number (and serial number) we're all just guessing here.