Hydraulic Pallet Stacker Servicing & Maintenance Questions

I bought this pallet stacker used, and have no experience doing maintenance on any lift system. Shortly after i received this, it started leaking hydraulic fluid from the release valve after use. It has slowly gotten worse. I also noticed that when I completely remove the release knob I get a huge blast of air and fluid from the pressure release. I appear to have lost enough fluid that I cannot lift anything over 1200 lbs. So here are my questions:
1. What kind of hydraulic fluid should I buy?
2. Do I refill it at the release valve?
3. What is likely wrong with the release knob?
4. If I need to replace the o-rings, what is the proper way to size them?

Any other help or advise you can offer will be greatly appreciated.

Is there not a lever in the handle that sets it to pump, neutral, release. That plug you have out is the oil fill plug I believe, judging by the manual for same unit on northern tools website. Yours looks different though so the inner part of that plug must seal the fluid in the inner cylinder and then opening it drains the pressurised fluid out to the outer cylinder where the oil is stored.

Just take the plug to the auto parts store and tell them you need new o-rings and a gallon of cheap hydraulic oil.

To me that is a weird cheap china lift, better one would have same handle as a normal pallet jack.

JP,

No, there is not a lever like a normal pallet jack has. The release appears to be the knob--turn it slightly to release the pressure.

Thank you for your help.