A heavy-duty cushion-tire forklift used in a high-speed cross-dock facility came in with a terrifying intermittent failure. The operator could drive around the yard for the first hour with perfectly stiff brakes. But after about an hour of heavy, fast driving and constant braking, the brake pedal would slowly sink to the floor. If they pumped the pedal rapidly, it would firm up again, but then fade away. The shop had already replaced the master cylinder and bled the system three times, but the problem persisted.
Because the pedal would firm up when pumped, everyone assumed air was getting into the system. But we couldn't find a single leak or a loose bleeder screw. We looked at the color of the brake fluid in the master cylinder reservoir. It wasn't the normal clear or light-amber color; it was a murky, milky brown.
Someone had used a high-pressure pressure washer to clean the forklift, and they had blasted water directly at the top of the master cylinder reservoir. The rubber diaphragm under the cap had a microscopic tear in it. The pressure washer forced water directly into the brake fluid.
Here is the critical physics failure: Water and brake fluid do not mix perfectly. Water is heavier than brake fluid. Over a few days of sitting, the water separated and sank to the absolute lowest point in the hydraulic system-the front brake calipers. For the first hour of driving, the brakes were fine because the water was sitting at the bottom of the caliper. But after an hour of heavy braking, the friction inside the caliper heated the brake fluid to over 300 degrees Fahrenheit. Pure brake fluid has a boiling point over 400 degrees. Water boils at 212 degrees.
The water at the bottom of the caliper boiled instantly, turning into steam. Steam is a gas, and gasses cannot be compressed. The expanding steam pushed the brake pads away from the rotor, pushing the brake fluid back up into the master cylinder, and the pedal sank to the floor. When the operator pumped the pedal, they pushed the steam bubbles back up into the cooler reservoir, where they condensed back into liquid, temporarily restoring the pedal. We replaced the master cylinder cap, flushed the entire system with fresh fluid, and installed new caliper seals that had been cooked by the steam. Never pressure wash the top of a brake master cylinder.
