In previous tips, we discussed cleaning a plugged metering port in the boom anti-drop valve to fix boom drift. But there is a much darker, more dangerous practice happening in the field that every mechanic needs to be aware of: mechanics who maliciously remove the internals of the anti-drop valve entirely.
Sometimes, a poorly maintained excavator will have a boom that drifts down aggressively. A lazy mechanic, not wanting to spend the time diagnosing the valve, will remove the anti-drop valve from the bottom of the boom cylinder, take out the spring, the poppet, and the seat, and put the empty housing back on. This turns the valve into a straight-through pipe fitting. The boom will no longer drift down because there is nothing restricting the flow anymore. The operator thinks the mechanic is a genius.
This is a death sentence for the machine. The anti-drop valve (also called a counterbalance valve) does two things: it holds the boom up, and it meters the speed at which the boom comes down. When you remove the internals, you remove all downward hydraulic resistance. If the operator is lifting a massive load ten feet in the air, and they accidentally bump the boom-down joystick-or if a hydraulic hose on the boom cylinder bursts-the boom will free-fall. It will not glide down; it will crash with the full, unrestricted kinetic energy of the heavy boom and the load. That violent impact will either snap the boom cylinder's mounting pins clean off the machine frame, or it will crush whatever is underneath it. If you find an excavator with the anti-drop valve gutted, refuse to work on the machine until the correct valve is reinstalled. It is an accident waiting to happen.
