A quarry operation reported a dangerous failure on a Volvo L90G wheel loader: the service brakes felt spongy and required the pedal to be pressed nearly to the floor, and the parking brake would not hold the machine on an incline. The machine was immediately pulled from the muck pile for safety. Because the L90G utilizes a fully enclosed, hydraulically applied multi-disc braking system, the fault pointed directly to a loss of stored hydraulic energy.
The brake system relies heavily on hydraulic accumulators to provide rapid fluid flow to the calipers, independent of the main pump's output. The technician connected a gauge to the brake accumulator test port. The pre-charge pressure-normally maintained by a nitrogen gas bladder at 900 psi-was reading zero. When the brake pedal was depressed, the main pump had to build pressure from scratch before the brakes engaged, explaining the severe lag and spongy feel.
The gas charging valve on the accumulator was checked for leaks, but it was dry. The technician then drained the hydraulic fluid from the brake circuit. Instead of standard ISO 46 hydraulic oil, the fluid that poured out was milky-white and had a distinct chemical smell. This indicated that the nitrogen bladder inside the accumulator had ruptured, allowing the high-pressure nitrogen to mix with the hydraulic oil, while simultaneously allowing fluid to fill the gas side of the sphere.
The loss of the accumulator's stored energy meant the brake system relied solely on the pump's flow rate. On an incline, when the engine was idling at low RPM, the pump could not generate enough flow fast enough to clamp the brake discs, causing the park brake to fail. The ruptured bladder was caused by a previous maintenance error: the system had been filled with the wrong viscosity hydraulic oil, which degraded the nitrile rubber bladder over thousands of cycles.
The repair involved replacing both brake accumulators with new units pre-charged to the factory nitrogen specification. Because the ruptured bladder had introduced nitrogen gas throughout the brake circuit, the entire system had to be bled meticulously. The technician cycled the brake pedal over fifty times while cracking the bleeders on the axle calipers, ensuring all aerated fluid was purged. The hydraulic tank was flushed and refilled with the correct, high-zinc hydraulic oil to protect the new bladders. Post-repair, the brake pedal was firm at the top of its travel, and the park brake held the fully loaded loader on a steep ramp.
