Here's a thread about your issue
Long story short is to remove and take it to a hydraulic shop and they can fix.
Or remove it and take it apart yourself and just take the end gland to the shop and have them match the seals. These older units should have standard dimensions and easy to match.
FWIW, I had a leaking cylinder on the backhoe/tractor. I had ordered a seal kit, readily available and was in the process of replacing the seals but I screwed one of the small stem seals. Took it to the nearby hydraulic shop, that I've used for hoses etc and showed it to one their tech, he pulled a seal off the shelf and put it in without any effort at all.