Most diesel pusher chassis RVs on the road today use a compressed air system. This is used as a source of energy to power things like the air brakes, air suspension and any other accessories that require air. But arguably, the two most important jobs of the air system are to run the air brakes and the emergency brakes. The purpose of an air brake system on heavy duty vehicles is to convert air pressure to mechanical energy to activate the foundation brakes. Federal Motor Vehicle Safety Standard (FMVSS) 121 dictates how this is to be accomplished for over-the-road vehicles.

With obvious safety implications of the braking systems, maintaining the air system on your coach is very important. It should be treated with the same caution as one would treat gasoline, high voltage or gunpowder. If released suddenly, compressed air could cause bodily injury.

There are many variations of manufacturers’ air systems, but most use the same components. Let’s look at some of the major components of the chassis air system — and the maintenance required for these components. Just remember to check out your compressed air systems with local experts before attempting any repairs or modifications on your own. Also, for more information you can visit spartanrvchassis.com/owners/training/rv-owner/.

Air Dryer
The dryer’s main purpose is to remove moisture and contaminants which are created while compressing air. The air dryer uses a desiccant cartridge to help remove contaminants in the air system. Most air dryers have a service interval set at two or three years to change the desiccant cartridge.

Air Gauges
Air gauges are required by FMVSS 121 so the driver can monitor the pressure in each system while seated in a normal driving position. There are two gauges: front and rear. The front gauge monitors the secondary air supply (front brakes and suspension) and the rear gauge monitors the primary air (rear brakes only). No maintenance required.

A pre-trip safety check should always be done before each trip to ensure you are operating a safe vehicle on the road. Checking for moisture in the air tanks should be one of the most important. All the major chassis builders supply a way to drain the air system using lanyards or drain valves to aid in removing moisture.

Air Tank Drain with Check Valve Test

1. Make sure both air gauges on dash are reading 70 psi or less. If gauges read higher, bleed the system down by pumping the brake foot valve until gauges read 70 psi or less.
2. Start the engine and run at high idle (1,000 rpm or higher) until air dryer purges in the back of the vehicle.
3. Make sure the chassis is safely supported before you turn the unit off. Jack stands are preferred. Park on a concrete or paved surface; it will be easier to see if any moisture is puddling under the air tanks.
4. Turn the key to the “Off” position. Turn the key back to the “On” position so gauges function and note the air pressure on both gauges. Do not restart the engine.
5. Locate the lanyards or drain valves for the air tanks. (Contact chassis manufacturer for location if not known.)
6. Drain the air completely from the “wet” or supply tank first.
7. Go into the unit and observe the air gauges. They should not read zero. This is to ensure the check valve between the supply tank and the primary tank is functioning properly.
8. Now drain the primary tank completely.
9. Check the air gauges once again. You should see the rear air gauge is reading zero and the front air gauge is not reading zero. This is to ensure the check valve between the primary tank and secondary air tank is functioning properly.
10. If any of the check valves fail this test, contact an authorized repair facility.
11. Now drain the secondary tank completely.
12. Check the air gauges again. Both front and rear should be reading zero.
13. Make sure all petcocks are closed and start the unit. Run at high idle until the air dryer purges. The air system should now be full and ready for operation.
14. If any of the above tests fail, contact an authorized service facility.

Finally, a note on automatic moisture ejectors. Automatic moisture ejectors are an aid in removing moisture. They are a mechanically operated valve. If you see wires going to the ejector, that is a heater coil for cold-weather operation. There is no indicator to tell you the ejector is not functioning.

By draining your tanks on a regular schedule, you will know if water is accumulating in the tanks. Knowing your air system is clean and dry will give you confidence — and will help you avoid a costly repair down the road. So don’t forget to drain your tanks!

Mike O’Neil started at Spartan RV Chassis — part of The Shyft Group family of brands and formerly known as Spartan Motors — in 1991, working on everything from production of motorhomes, fire trucks and bus chassis to quality control and customer service. He served as a technical trainer at Cummins Michigan from 1999-2001 but returned to Spartan in 2001 as a technical trainer for the Spartan Training Academy, a comprehensive training program that gives RV owner’s the “ins-and-outs” of coach driving, maintenance and service needs. The curriculum includes instruction from award-winning certified technicians/trainers and includes extensive maintenance instruction, driver training and review of the chassis sub-systems such as electrical and air suspension.

In addition to training coach owners for more than 25 years, Mike has also trained thousands of motorhome and fire truck technicians. He often represents Spartan RV Chassis at major events and rallies to present seminars for OEM and industry support.