An aggregate processing yard reported that its SDLG LG958L wheel loader experienced progressively worsening brake performance. When descending the ramp from the stockpile with a full bucket, the brake pedal felt "hard but ineffective," and after several consecutive stops, braking force almost disappeared entirely for a minute before recovering. Additionally, the air dryer purge valve was spitting an oily mist every time it cycled.
The technician began with the air system, since the oily discharge was a clear clue. He drained the "Wet Tank" (primary air reservoir) and found a substantial quantity of dark engine oil mixed with the condensate. Oil in the air system can only come from one source: the engine-driven "Air Compressor." He removed the compressor discharge check valve and found it coked with carbon and oil residue. However, further investigation revealed the deeper root cause. The compressor's "Unloader Mechanism," which is controlled by the governor to hold the compressor inlet valve open when system pressure reaches cut-out pressure, was seized in the "loaded" position. Instead of freewheeling unloaded, the compressor pumped continuously against the dryer, running hot. The excessive heat broke down the compressor piston ring sealing, allowing crankcase oil to be pumped into the air lines. The oil then coated the "Air Dryer Desiccant Beads," destroying their moisture absorption capability, which explained why water was also accumulating in the tanks. Water in the brake booster was the cause of the fade.
The brake booster (air-over-hydraulic unit) was then disassembled. The large rubber "Power Piston Diaphragm" was found swollen and partially delaminated-classic damage from prolonged contact with emulsified oil-water mixture. The swollen diaphragm was sticking to the booster housing walls at high application rates, momentarily blocking the air passage and causing the intermittent fade.
The repair involved replacing the air compressor with a remanufactured unit (the rings and unloader were unserviceable), replacing the air dryer cartridge, and flushing both air reservoirs. The brake booster received a complete diaphragm and seal kit. All brake hydraulic fluid was replaced due to suspected moisture contamination. After the repair, brake force remained consistent through 20 consecutive ramp stops, and the air dryer purged clean, dry air only.
