Broken Boom Lift. Diagnosing & Rebuilding The Master Cylinder. JLG 80 HX

When you work with old equipment, there are always going to be problems that pop up. Here, my boom lift develops another one involving the self leveling system, and I show how I diagnose and fix it up.

Self leveling system explanation:
Picture the hydraulic tank to the pump to a control valve. When the valve is actuated it sends pressure down lines that are connected to both cylinders. One line is pressurized the other is return oil. (They swap pressure vs return when the valve is moved the other way.) The control valve when closed isolates the 2 cylinders from the rest of the system. The 2 cylinders are basically in parallel. Both extension chambers are connected, and same with the retraction chambers. So let’s say we are moving the boom down and the control valve for the basket level cylinder is closed. The boom will push the master cylinder down making pressure in the extension chamber. That oil will flow to the slave cylinder and cause the rod to extend and proportionally moving the basket with the amount of boom movement. Incidentally, the oil in the retraction chambers also flows, from the slave to the master. The amount of oil in the system is fixed as long as the control valve is closed and it will maintain its level. Now when you command the basket to tilt, the control valve opens and pumps more oil into one side and allows the other side to return, moving the basket. Then when the valve is closed again the system will maintain that new level.
Also worth noting, when the basket tilt valve is opened, and pressure goes to the leveling system, it’s true that the master cylinder is now pushing or pulling in the boom a little bit. But it’s nowhere near enough force to move it since the main cylinder is there.

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