An explanation of how the system works:
Two lines come off the main control valve and lead to what I will call a balancing valve mounted on the cross support bars forming the X
From the bottom of that valve the 2 lines feed each cylinder,
at each cylinder from the balancing valve, pressure is maintained equally when the control lever is in neutral( as long as there is no air in either side), no leaking past the bypass check valve located inside the balancing valve, and no leak in either cylinder, any of which could cause a loss of pressure on one side.
When you move the lever to swing right, you send pressure through one side of the balancing valve to the cylinder (on your right side as you are sitting in the seat),
that same pressure inside the balancing valve while your swinging right moves a valve inside the balancing valve allowing pressure from the left cylinder to be sent against the closing force of a check valve with a heavy spring holding it closed.
The force from the right cylinder as you swing right pulls the left cylinder with the chain compressing the fluid in it, pressure then builds up on the left cylinder forcing the check valve open allowing the fluid in the left cylinder to bleed past it back to the opposite line coming from the main control valve pressure is not being applied to
During the process of swinging right, although the fluid in the left cylinder is being released, a ( lower than main hydraulic pressure) is being maintained in the left cylinder by the spring holding the check valve closed. This allows a lower but constant pressure in the left cylinder as the main line pressure is being applied to the right cylinder, therefore keeping the chain tight
If your chain is not tight, you are losing pressure on the side that is loose as you are pulling the opposite direction.
To troubleshoot, first see if there is air in the loose side cylinder, there are 2 bleed valves on the balancing valve, one on each side for each cylinder . Note since the lines from the cylinders to the valve cross, the right bleed valve looking rearward bleeds the left cylinder, and the left bleed valve bleeds the right cylinder.
Bleed the side where the hose from the loose cylinder connects.
If you are sure there is no air in the loose side of the system, it means you are leaking fluid pressure either inside the loose side cylinder, or inside the balancing valve.
Since there is only one check valve releasing pressure from the cylinder that is being pulled as you swing, a leak past the check valve would cause either opposite cylinder to be lose as you swing either left or right.
Unhooking the lines and letting the backhoe sit idle is most likely to cause air in your lines. Bleeding would be the first thing to try, then checking for pressure loss on the loose side cylinder and if found changing seals, if no problem exists there, remove the balancing valve and pressure check it as directed below, if it does not hold pressure disassemble it and clean it while checking for wear and changing any seals.
If you can do a bench test, rig up a way to apply pressure to the cylinder, even air pressure will work, and if you do not loose pressure, and you previously eliminated air in the system being a problem by bleeding it, your problem has to be in the balancing valve.
To bench test the balancing valve remove it, rig up a way to pressurize the port leading to the cylinder that the chain is lose on, Both TO CYL) ports to going each cylinder should hold pressure bench testing.
It is basically this simple, while the balancing valve is sitting idle with no input pressure to either top port on the balancing valve which are labeled with an arrow and the words To Valve, both bottom ports with arrows labeled To CYL should hold pressure applied to them. If they don't the valve is leaking inside.
If the cylinder does not hold pressure, it has a leak inside.
