An air brake compressor is the heart of a heavy truck's air system. Typically bolted to the engine gear train or driven by a belt, this piston pump runs constantly while the engine is turning. It draws in filtered air from the engine intake, compresses it, and pumps it through a cooling line to the air dryer and storage reservoirs. It supplies the pressure needed for brakes, air suspension, cabin seats, and horn accessories.
The compressor is lubricated by engine oil and cooled by engine coolant. Over time, the internal piston rings wear down, letting engine oil slip past into the air lines. This is known as "oil carryover." The hot oil mixes with moisture, forming a sticky sludge that clogs air valves and ruins brake diaphragms throughout the vehicle.