A construction rental company reported that a Genie GS-2669 rough-terrain scissor lift was raising its platform slowly - the raise cycle that previously took 35 seconds now took over 90 seconds - and the platform would gradually drop by 10 to 15 centimeters over a 10-minute period while the operator was working at height with no manual lowering input. Both faults worsened when the machine was carrying a heavy load near its 680-kilogram platform capacity.
The technician first verified the hydraulic oil level (correct) and inspected the hydraulic pump's drive system (the machine's pump was belt-driven from the auxiliary engine, and the belt showed correct tension). He then performed a flow test at the lift circuit's supply line, measuring 18 liters per minute with the engine at operating speed - within specification. This confirmed the pump was delivering adequate flow, isolating the fault to the lift cylinders or the hydraulic control valve.
He then performed a "Cylinder Internal Bypass Test." With the platform raised to mid-height and the lift circuit dead-headed by a manual shutoff valve, he measured the platform's descent over 5 minutes. The platform dropped 12 centimeters - far beyond the 1-centimeter allowable drift. This confirmed internal bypass: hydraulic fluid was leaking from the pressurized side of the lift cylinders to the return side, allowing the platform to descend without any external valve command.
He removed both lift cylinders and disassembled them on a bench. The "Piston Seal Assemblies" - pressure-energized polyurethane U-cup seals with backup rings - were visibly worn, with the seal lips hardened and flattened. The cylinder bores showed scoring along the lower quadrant, consistent with past contamination ingress. The rod gland seals were also worn, allowing slight rod-side oil weepage during extension.
The root cause was traced to hydraulic contamination. The machine's return filter had exceeded its service interval and had partially collapsed internally, allowing bypass flow of unfiltered hydraulic oil back to the reservoir. Fine particulate contamination had circulated through the system, progressively abrading the cylinder seals and scoring the cylinder bores. The contaminated oil had also lodged a small particle in the lift circuit's "Lowering Brake Valve" - a pilot-operated check valve that prevents free platform descent when the hydraulic supply is off - causing it to seat imperfectly and produce the slow, gradual drop under static load.
The repair involved honing both cylinder bores to remove the scoring, replacing both cylinders' piston seal kits and rod gland seals, replacing the lift circuit's lowering brake valve cartridge, and replacing the collapsed return filter with a new unit. The hydraulic system was flushed with a portable flushing rig to remove circulating contamination, and fresh specified hydraulic oil was installed. After reassembly and bleeding, the platform raised at the correct 35-second cycle, and the static drop over 10 minutes measured under 1 centimeter. The return filter was added to the machine's 250-hour service checklist to prevent recurrence.
