A hydraulic cylinder is essentially a glorified automotive shock absorber. Shock absorbers work passively as the cylinder reacts to road bumps and uneven surfaces. The cylinder moves up and down using hydraulic fluid to flow between valves that absorb the shocks. Hydraulic cylinders use hydraulic fluid in an active way, pumping fluid through the valves to extend or contract the cylinder. As such, large, heavy loads and pressure can be manipulated, making hydraulic cylinders well suited for lifting things like crane arms and tractor buckets, or using the captured pressure in a hydraulic cylinder on a log splitter, to split wood.
Fill the hydraulic cylinder with hydraulic fluid. Every hydraulic cylinder has a specifically labeled fill hole that must be topped off before proper bleeding can take place. Use your adjustable wrench to remove the filler nut, then pour in hydraulic fluid until it is level with the top of the hole. Replace the filler bolt tightly.
Extend the hydraulic cylinder fully.
Locate the bleeder nut, and open it up with the cylinder fully extended. Many bleeder nuts can be loosened by hand, but some require an adjustable wrench. Loosen the nut until the cylinder begins to retract on its own.
Observe any discharge from the bleeder hole. Air will first appear as the cylinder retracts, so allow this procedure to continue until hydraulic fluid begins to squirt out of the bleeder hole. When there are no longer any air bubbles present in the squirting fluid, tighten down the bleeder nut.
Wipe off all excess hydraulic fluid with a rag.