By Russ and Tiña De Maris
If you hear the word “Westinghouse,” your mind probably jumps to kitchen appliances, and you’d be right. But George Westinghouse, founder of the company that later produced appliances, can also be remembered as the inventor of air brakes. First designed for trains, air brakes later moved to heavy trucks, and today they’re in many large motorhomes.
If you’re thinking about buying a big Class A motorhome, you may be faced with the choice of a rig with what most would call “normal” hydraulic brakes, or a unit with air brakes. For some who are unfamiliar with air brakes, the feeling they get is insecurity, associating air brakes with runaway rigs. Actually, in terms of braking safety, a rig equipped with air brakes is likely to be safer than one equipped with hydraulics. Why this is so is best understood by comparing hydraulic to air systems.
In your car or pickup, when you step on the brake, liquid hydraulic fluid is pressurized, flows through a system of brake lines, and typically actuates a piston, forcing it and a caliper to squeeze down on a brake disc or rotor. A brake pad actually makes contact with the rotor. There will be a rotor for each wheel of your vehicle, and this act of squeezing slows the vehicle or brings it to a stop. In some vehicles, it’s a little different. Instead of hydraulic fluid actuating a piston to clamp down on a spinning disk, the piston instead forces brake “shoes” equipped with pads to open up against a spinning drum (again, one per wheel) causing the slow-to-stop action.
In an air brake system, instead of hydraulic fluid doing the pushing, as it were, pressurized air actuates the calipers or the brake shoes in the system. However, one major difference comes into play. In the case of the rear brakes, until air pressure is applied those brake shoes or calipers are in the fully extended or “braked” position. Until you start up the rig’s engine and sufficient air pressure is developed, the rear brakes of the rig are locked up.
This makes for a much safer system. In a hydraulic system, if the brake system were to develop a leak of fluid, a small amount of reserve fluid is present. However, once the brake fluid is gone, you can no longer actuate the brakes. If you’re on a steep downgrade and lose your brake fluid, you have a serious problem.
In an air brake system, a compressor is continuously producing a volume of air – if a small leak were to develop in the brake system, the compressor would have no problem overcoming it. Ah, but what if the engine were to conk out and the compressor stop? First, you still have a fairly large reserve of air, and even if that were depleted, once the air pressure is gone the rear brakes would lock up, slowing and eventually stopping your rig. Meantime, the front brakes would NOT lock up, allowing you to safely steer your rig.
Some wonder, though, if a special driver license endorsement is required for an air brake rig. Commercial drivers would see this as a requirement, but it’s not the case for most U.S. RV drivers. Check with your home state DMV for details. Braking an air brake system is somewhat different than a hydraulic system, but it’s not a difficult skill to master. However, it is important to understand how to test, operate and maintain the system. And remember, air brakes are no different than hydraulic brakes in terms of how well they work (or don’t work) if you “ride the brakes” on a steep descent.
All in all, air brakes are nothing to be feared. If anything, give George Westinghouse credit – not just for your refrigerator, but for making the roads a bit safer.
[Updated 8.14.2017 to clarify DMV requirements for drivers]