Fisher, if the tank valve stem is leaking and you have not even connected it to the forklift, you might need to have a new valve installed in the tank.
But.......because the fuel is in liquid state at the valve area, a fuel leak that only lasts a couple of minutes (after closing the valve hand ****) or so may only be due to some wear in the valve packing and/or valve stem. If the leaking continues for more than a few minutes after closing the valve, you probably have a defective valve assembly. If the leaking stops a few minutes after closing the hand **** you may be able to tighten the packing and stop this leakage.
Look directly behind the hand wheel **** and see that there is a packing gland nut that the valve stem goes through.
After LP cylinders have been in use for a long time, the valve stem and the packing material inside the valve can develop some wear, resulting in a leak as you describe.
I suggest you try to tighten the packing nut about 1/4 turn (clockwise) at a time and recheck for a leak.
If......you are unable to stop the leak in this manner (as is the designed method) then you need to have the valve replaced.
This would require an empty (and purged) tank and is best left to a propane distributor service facility as I believe there might be some certification details to be addressed in cases of valves and such being replaced.
In the very least you would need to purge the air that will enter the tank while the valve is out before refilling the tank completely.