When a main hydraulic piston pump begins to fail, it doesn't usually explode all at once. The high-pressure swashplate and valve plate slowly wear against each other, shedding microscopic particles of bronze and steel into the hydraulic oil. By the time the pump loses enough pressure to derate the machine, the damage is catastrophic and the pump is un-rebuildable.
There is a way to catch this failure weeks before it happens, and it requires paying attention to the pilot filter. The pilot system is a low-pressure, low-flow circuit that operates the joysticks and the main control spools. The pilot pump draws its oil from the main hydraulic tank. Because the pilot filter is a fine-mesh, low-flow element, it catches debris that the massive, high-flow main return filters might miss or bypass under high pressure.
Every 500 hours, when you change the pilot filter, do not just throw it in the trash. Take a razor knife and slice the filter media open lengthwise. Wipe the inside of the media on a clean white paper towel. If you see a metallic bronze or silver sheen on the paper, the main pump's rotating group is actively shedding metal. The pilot system is acting as the canary in the coal mine. If you catch that sheen early, you can pull the main pump, re-lap the valve plate, and replace the pistons for a few thousand dollars before the metal circulates through the entire system and destroys every cylinder and control valve on the machine.
