Tuesday, December 6, 2022


Are air brakes for you?



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]


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Bill Morgan
3 years ago

This article contains a fundamental error. The system described, which uses springs to apply the rear brakes, is a Spring Brake system, not Air Brake. With Air Brakes the brakes are applied with air pressure, so if you lose pressure you have no stopping power. Check the original brakes on a GMC PD4104, 4107, 4903, 4905 for reference.

Guy Jackson
1 year ago
Reply to  Bill Morgan

You are describing a vehicle with air assist brakes (a hydraulic brake system that is assisted by air pressure rather than hydraulic pump). If your vehicle has brake fluid, it does not have air brakes.

Air brakes are applied by springs until there is sufficient air pressure to force them off. Loss of air pressure will cause the brakes to be applied. When you first start a vehicle with air brakes, it cannot move until air pressure reaches at least 60 psi.

Louis Frank
5 years ago

Statement in article that no endorsement required for rv air brake drivers is incorrect. If you live in PA you take a cdl test and get a class B endorsement.

Ron Twellman
5 years ago

Just so the non-mechanically inclined among us aren’t alarmed by the statement in this article about a leaking hydraulic system, they need to know that modern hydraulic brakes have a dual master cylinder system that separates the front and rear brakes. If you have a leak in one system, the other will continue to operate. Your brake system warning light will also come on to let you know that it’s time for some serious service work, but you may already have been clued in to that by the fact that it took an extra amount of time and distance to make that last stop!

Let’s all be safe out there

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