My guess is Hynautic. I have an 83 Californian with Hynautic. Look for a reservoir with an air valve on top similar to a tire valve.
There's probably no reason to buy a new pump, they're expensive. A good hydraulic shop can overhaul it for you. I recommend you think about overhaul for both helm pumps, the ram and the autopilot pump. Also a complete drain, flush and refill of the system. While you're at it a close inspection of the hydraulic lines too. 35 yrs is a long time.
Some things are worth doing simply because they are worth doing.