A skid-steer loader came into the shop with a textbook hydraulic failure: the lift arms would barely raise an empty bucket, and the auxiliary hydraulics couldn't spin a small auger. The most obvious symptom was a horrific, high-pitched whining noise coming from the back of the machine the moment the operator touched the joysticks. The shop assumed the gear-type hydraulic pump had internally failed and was cavitating.
We hooked a test gauge to the case drain line of the pump. Case drain flow should be near zero; if it shoots high, it means oil is blowing right past the worn internal gears. The case drain read zero. The pump was building full pressure, but the oil wasn't making it to the cylinders. We then checked the pressure right at the pump outlet, and it was pegged at 3,500 PSI-the relief valve setting. The pump was dead-heading.
We went straight to the hydraulic tank and pulled the suction strainer-the coarse metal screen inside the tank that the pump pulls oil through. We held it up to the light. It looked perfectly clean. But then we noticed the center of the screen was pushed inward, forming a concave shape.
What had happened was a massive internal failure of a hydraulic cylinder seal upstream. Chunks of shredded polyurethane seal had washed through the return lines and completely coated the outside of the suction filter. Because the pump pulls a strong vacuum, it literally sucked the filter media inward, collapsing the metal screen like a crushed soda can. The oil couldn't get through, so the pump sucked air from a loose fitting upstream, causing the whine, and dead-headed against the relief valve. Because the filter looked visually clean from the outside, the mechanic initially ruled it out. It's a vital rule: if a pump is whining and making no pressure, always physically pull the suction filter and make sure the center hasn't collapsed.
