An excavator operator complained that when he lifted the boom and simultaneously curled the stick in toward the cab, the stick would suddenly droop down on its own. It was a dangerous, jerky movement. The mechanic replaced the stick cylinder piston seals, assuming they were bypassing under combined load. It didn't fix the droop.
The problem was in the main control valve, specifically the "regen" circuit for the stick. When an operator booms up and curls the stick simultaneously, the rod-end of the stick cylinder is retracting, and the bore-end is extending. The bore-end needs oil faster than the pump can supply it. To prevent cavitation, the stick spool has a built-in regen valve. It takes the oil being pushed out of the rod-end (which is shrinking) and redirects it back into the bore-end (which is growing) to supplement the pump flow.
Inside this regen circuit is a tiny check valve. A microscopic piece of debris-a sliver of Teflon from a neighboring seal-was holding the regen check valve cracked open. When the operator used the stick alone, the pump pressure was high enough to overcome the leak. But when booming up simultaneously, the pump flow was divided. The pressure dropped just enough that the pump couldn't overcome the open regen check valve. The bore-end pressure bled back into the rod-end, causing the stick to lose holding force and droop. Cleaning the regen check valve spool cured the ghost droop instantly.
