I have an old engine hoist. It's the type with removeable legs that store upright. It's Taiwan, not H.F. It had a leak earlier in this year in the relief valve. I replaced the rubber ring and stopped the leak but the ram would no longer move up when I operated the pump.
I checked around and after reading on the subject on the internet, I discovered how to get it working again.
After pulling the ram out by hand, I made sure that it was chock full of oil up to the filler cap (I know, that's not how you fill one.....more on this in below).
Then I removed the ram and took it outside and put papers down on the ground to catch the oil. Then, with the unit full, the piston all the way out and the unit laid horizontal with the relief valve facing up, I unthreaded the relief valve from the base and set it aside.
Then I slowly pushed the ram into the body. I got some oil out, but more importantly, I got a blast of air and oil that apparently purged the air that was stuck inside. After pushing a little more to be sure that there was no more air, I threaded the relief valve back in, wiped up the mess and reinstalled the ram in the vertical position on the hoist.
I then opened the relief valve and lowered the ram all of the way. I then filled the ram with oil to the level of the filler cap. It now works perfectly.
Since then, I had the exact same issue with my 20 ton Taiwanese hydraulic bottle jack press. Once bled with a new O-ring on the relief valve, it now works as well.
The trick is to get the ram horizontal first so that the air can rise and then purge it out through the relief valve opening.
It works for me.