I've got a 1979 Hyster H40H, LPG model. A few years ago, occasionally, there would be be no hydraulic steering or lift. Turning the steering wheel rapidly would most times trigger something and the steering control would return and at the same time, so would the lift. This problem would eventually get worse and the solution to getting steering and lift, was to loosen the hydraulic hose fitting on the back of the pump and while the engine was running, start turning the steering wheel until what seems to be air bleed from the system as fluid would then start to leak at the fitting as the steering and lift returned. Tighten the fitting and away you go. Sometimes, you'd start the truck and no issues, steering and lift working, other times, no steering or lift.

Someone suggested I check the filter in the tank. I recently had the filter housing completely apart, cleaned the smaller diameter strainer, replaced the filter, "O" rings and gasket and new hydraulic fluid, but same problem exists. Seems like the pump is sucking in air somewhere.

Would appreciate any suggestions as to where to start at trying to diagnose what the problem might be.