Applying Service and Spring Brakes

Air Brake Architecture: Generating, Storing, and Cleaning Air

Commercial vehicles equipped with air brakes rely on three distinct systems working together: service brakes, parking brakes, and emergency brakes. The service brake system applies and releases the brakes when the driver uses the brake pedal during normal driving. The parking brake system applies and releases the parking brakes when the driver operates the parking brake control inside the cab. The emergency brake system uses parts of the service and parking brake systems to stop the vehicle if the brake system fails.

The supply side of the system centers on the air compressor, which pumps compressed air directly into the air storage tanks. The engine drives the air compressor through mechanical gears or a v-belt. If the air compressor is designed with its own separate oil supply rather than using engine oil, the driver must check the compressor oil level before driving. The governor controls when the compressor pumps air into the storage tanks, stopping it at around 125 psi and allowing it to pump again at around 100 psi. The governor stops the compressor from pumping air once storage tank pressure reaches the cut-out level of around 125 psi. When tank pressure falls to the cut-in level of around 100 psi, the governor allows the compressor to start pumping again.

Air storage tanks hold enough compressed air to allow the driver to use the brakes several times even if the compressor stops working entirely. Compressed air usually contains some water and compressor oil, which tend to collect at the bottom of the tanks. Water in an air brake system can freeze in cold weather and cause brake failure. Manual air tank drain valves allow drivers to clear these contaminants by turning a valve a quarter turn or by pulling a cable. Drivers must drain manual air tanks completely at the end of each day of driving. Vehicles may instead feature automatic air tank drain valves that expel water and oil automatically, and these automatic tanks can be equipped with electric heating devices to help prevent freezing in freezing weather.

To provide extra cold-weather defense, some vehicles feature an alcohol evaporator. This device puts alcohol into the air brake system to reduce the risk of ice forming inside valves and other delicate mechanical parts. Even on vehicles equipped with an alcohol evaporator, daily air tank drainage is still strictly required unless the tanks have automatic drain valves. Finally, a safety relief valve is installed directly into the first air tank that the compressor pumps air into. This safety relief valve protects the air tank and the entire brake system from excessive pressure, and it is usually calibrated to open at 150 psi. If the safety relief valve ever releases air, something is wrong with the system and the vehicle must be repaired by a mechanic.