Air Brakes...everything you wanted to know but didn't know who to ask

Air Brakes: Everything You Wanted to Know but Didn’t Know Who to Ask

by Brian Keys, former technical services manager at Country Coach Inc, Autumn 2002

 

*Though authored some time ago, it provides timely advice on your motorhome's air brakes 

Over the years we have found that many discussions with customers regarding things like horsepower and torque curves on the diesel engines which Country Coach uses ends with "what manufacturer installs the largest engine to climb the steepest grade known to man?" Why not ask about going down the other side and the braking capability of these large diesel pusher homes on wheels? The answer is that everyone knows that the brakes usually work and that is the end of their concern. 

Others not only want the brakes to operate flawlessly, we also want to know how they work. Country Coach Inc. uses air brakes on all of its diesel pusher motorhomes for three basic reasons:

1) Stopping Capabilities

2) Reliability

3) Ease of Maintenance.

Stopping capability is measured in the number of feet required to bring your coach to a complete stop from any given road speed. The engine exhaust brakes used on Country Coaches are wonderful devices designed to slow your coach and/or assist your air brakes, but they will not stop your coach completely. When referring to motorcoaches that weigh in excess of 30,000 pounds, air brakes are the logical and preferred method to stop this much weight. Automatic slack adjusters that continually keep the brakes properly adjusted and ABS (Automatic Braking System) enhance the stopping capabilities of air brakes. Country Coaches are equipped with both of these enhancements.

Reliability is almost legendary on air brakes since they have been used extensively in the commercial trucking and bus industries for over 50 years and millions of miles. Eighty to one hundred mile service intervals are commonplace for long haul commercial trucks when they adhere to the prescribed lubrication and maintenance procedures.

To understand the maintenance required on air brakes. It is also necessary to understand their components and function. The heart of the system is the engine mounted air compressor. This is the same unit that supplies air to your air ride suspension and air-operated accessories like air horns, for example. this compressor is governed and operates between 90 to 125 PSI. The compressed air is then stored in the primary and secondary air tanks for use as needed by the air brakes, suspension, and accessories. During engine operation, the compressor continually replaces the air supply because when the air brakes are applied the air suspension is leveling the coach, large amounts of compressed air are used for these functions. The air brakes are applied via mechanical "S" cam and lever mechanism that is attached to an air canister at each wheel. The proportioning, or treadle, valve under the brake pedal is activated by pushing on the brake pedal. How hard you push determines the braking force once applied.

The maintenance required on air brakes other than the periodic greasing of the lever mechanism is best left to the professionals at the Country Coach Service Center, or a local truck repair facility. The air brake components are large, heavy-duty items and require some very hefty tools that most of us do not carry around in our coaches.

The braking characteristics of air brakes on a large motorcoach are not that much different than those on most power-assisted brakes on your car. A light touch is all that is necessary. With very little practice, you can become a pro. However, always remember these motorcoaches weigh 10 to 15 times as much as your tow car. You should judge your traveling speed and stopping distance accordingly.

The release of compressed air that you hear when the primary and secondary tanks are full is a normal function of an air brake system. This can be somewhat alarming if you don't know what it is. Standing outside your coach with the engine running when this air release occurs has made many owners look at each other and say, "What was that!"

Some very experienced coach owners feel they have terrible air system leaks if they lose any air on the dash-mounted air pressure gauge overnight. This is an unnecessary concern since all air systems use air and a small loss is expected and normal. The Dept of Transportation states that a two-pound loss of air per minute with the engine not running is within normal specifications.

With a Country Coach it is also normal to require up to five minutes for your air system to be refilled if your coach has been stationary for a period of time or if the air suspension has had the air dumped during the leveling process. This refilling procedure is best accomplished by running the engine at high idle for approximately five minutes with your air leveling system in the travel mode. Even though your travel light comes on when you turn off your HWH leveling control, that does not mean your air system is refilled and ready to travel.

The proper way to ensure air brakes and complete air system have refilled and are ready for operation is to observe the air pressure gauge on the dash and be sure it reads in the 100-120 PSI range. For your clarification, 90-125 PSI pounds range mentioned previously is the operating range of the engine mounted air compressor. During coach operation the pressure gauge will be in the 100 to 125 range on the gauge in the dash.