Air Brakes: Preventative Maintenance

Every state in the U.S. has specific guidelines for operating a vehicle with air brakes. The tests to obtain a commercial driver’s license are demanding, as are the steps to maintain such a vehicle. Here are some steps you'll want to take before heading out on the road:

  • Make sure the minimum operating pressure for a vehicle air-brake systems is no less than 85 psi (pounds per square inch) for a bus and 100 psi for a truck.
  • Check that it takes no longer than two minutes for air pressure to rise from 85 psi to 100 psi at 600 to 900 rpm. (This is called the air pressure buildup rate.)
  • Confirm that the correct cut-out governor pressure for the air compressor is between 120 psi and 135 psi. Cut-in pressure is 20 psi to 25 psi below cut-out pressure.

You'll also want to watch for water in the air-brake system, a byproduct of the condensed air. Air-brake lines don’t like water, especially in colder climates where ice can block air from reaching the brake mechanism and cause the wheel to lock up. To prevent this problem, many of the modern systems have automatic drain valves installed in each air tank.

truck accident
Andy Sacks/Getty Images
Poor maintenance of air brakes can lead to accidents.

Air couplers can also pose a problem. Worn rubber seals will cause air to escape. While the compressor can overcome a small leak, running compressors too hard can lead to failure. Again, as we’ve learned, air loss isn’t necessarily a bad thing, but it will mean you’re stuck. For truck drivers, getting stranded in the middle of a mountain pass probably isn't on the itinerary.

Poor maintenance leads to runaway truck.
On April 25, 1996, a 1988 Mack cement truck collided with a small Subaru sedan in Plymouth Meeting, Pa. As the driver of the cement truck approached an intersection at the end of a downhill off ramp, his brakes failed and the truck barreled into the intersection, striking the Subaru and killing its driver. The National Transportation Safety Board investigated the incident and found several problems with the truck, notably reversed brake lines and a secondary system failure. Those two issues left the truck with only an estimated 17 percent to 21 percent of its total braking capability. Unfortunately, the driver had no idea he had a brake failure. Poor maintenance resulted in a senseless death that could have been avoided. [source: NTSB]

Brake sensitivity, another byproduct of air brakes, can lead to accidents, especially for inexperienced drivers. Air-brake systems are designed to work on vehicles carrying heavy loads. Have you ever wondered where all those dual skid marks on the expressway come from? That’s a product of light or empty trailers locking their rear wheels. Probably the worst fear for a truck driver is jackknifing. It’s never good when the back end of the trailer creeps up alongside the cab. Trucks traveling in rain and snow can easily jackknife if too much brake is applied.

Most modern vehicles with air brakes use a dual system. In essence, such equipped vehicles have two systems in case one should fail. Anti-lock brakes can now be found in tractor-trailer rigs and work much the same way as ABS systems found in passenger cars.

Fundamentally speaking, air brakes are efficient and reliable. However, don’t hold your breath if you’re hoping to find them in your car any time soon. Air-brakes systems occupy too much space and attention to be considered practical in cars. Just look at a Peterbilt truck as it saunters down the interstate. Have you seen the big tanks tucked behind the fuel tanks? Try finding a place for those under the hood of a Honda Civic.

If you’d like to learn more about air brakes and read some related HowStuffWorks articles, explore the links on the next page.