Sure, the lift will work fine, as long as you have enough resistance in the valve and cyl to build up pressure. Any leakage will decrease you lifting capability.
Ask your dealer what the leak down rate is for your tractor. That is only when new. Over time the spools in the valve, and the seals in the cylinders will wear or deteriorate. It usually amount to , what is acceptable to you. You could rebuild cyl, and get a new valve.
Pilot operated check valves is the only other recourse for safety related work, as in using the bucket as a work platform. They are designed to lock the cyl, as long as the cyl are good.
G'day taking into account the age of the tractor you won't fix it if it only leaks down when stopped it is a common complaint on most massey's of this and earlier vintage sorry but you just gotta learn to live with it