Being that old, the 3pt cyl seal might be worn, cracked or brittle.
It could be a worn valve, but it is probably the cyl seals that are leaking.
Try this, with a load on the 3pt, raise the 3pt, turn the knob to lock the 3pt for transport and wait 24 hr and see if the arms have dropped. This will tell you the condition of the seals.