A wheel loader working in a steep quarry was brought in with a terrifying symptom. It operated perfectly on flat ground, but when driving down a 10% incline with a full bucket, the operator would apply the service brakes and the pedal would instantly go soft, dropping to the floor with zero braking force. After resting for five minutes, the brakes would return to normal.
The machine used a fully enclosed, oil-cooled wet disc brake system. The brakes themselves were fine; the friction plates had plenty of material. The problem was in the dedicated brake cooling circuit. A small hydraulic pump continuously circulates cool oil from a reservoir through the brake housings to carry heat away.
We traced the suction line from the brake cooling pump to the reservoir and found a fitting with a microscopic crack. On flat ground, the pump had a positive head of oil and drew fluid easily. But when the machine pointed downhill, the oil in the reservoir shifted away from the suction fitting. The pump, now starved, pulled air through the micro-crack, causing severe cavitation. It stopped flowing oil entirely. Without coolant flow, the thin film of oil trapped inside the brake housings boiled instantly under the heavy braking load. The boiling oil turned to vapor, which is highly compressible. The operator pushed the pedal, compressed the vapor, and got no clamping force. When the machine sat level, the oil returned, the vapor condensed, and the brakes felt solid. Replacing the cracked suction fitting fixed the phantom brake failure.
