well then, what i would do is make up a gate valve hose assembly and plumb it into the main hydraulic line going from the control valve out to the mast. Once you get it connected raise the mast in the air and close off the gate valve. If you still have drift then you will know the problem is not in the control valve and is indeed in the mast cylinders.
At that point you will have to chain up the carriage and mast channel that is in the air so it will not fall. Remove the lines at the base of each cylinder and check the fitting, it should be a check valve (velocity fuse) and see if it is damaged. If you find no problems there then the cylinders will have to be removed and dissassembled and checked for damage.
You will probably find either a bad balance valve in the piston or bad packing. One of those cylinders can be bad but cause both to drift since they are tied together.
good luck
2 stage mast?
how many cylinders total? just one on each side? or does it have one in the middle too?
some of the most common mast drifting issues are:
- bad piston packing seals
- faulty valve in cylinder (safety valve)
- faulty check valve in control valve
- faulty valve section
troubleshooting hydraulics is fun..... NOT ;o)