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Motorhome air brake system testing

by Jerry Fitzgerald

This test identifies the upper and lower limits of the air pressures. Start the engine and allow the air pressure to build up until it reaches its maximum and the compressor turns off (this should be no more than 130 pounds of pressure). You may hear the spitter valve pop when the maximum pressure is reached. Then depress the service (foot) brake several times. Each time you depress the service brake pedal the air pressure will go down and you can see the pressure go down on the air gauge. Wait about 2 seconds between each time you depress the brake pedal in order to see if the compressor restarts and the air gauge needles start going up again. Watch the air gauge and keep doing this until the compressor restarts. The compressor should restart at or above 85 pounds of air pressure. Now you know the maximum air pressure in your system and the air pressure where the compressor restarts. NOTE: There are two needles in the air gauge. One is for the front air tank and one is for the rear air tank. They should be close together, no more than 2-4 pounds apart. If they diverge too much it may indicate a problem with one of the air tanks.

This test checks for leaks in the air tanks. Continuing from the AIR GOVERNOR TEST above, the air pressure should be at its maximum for this test. You should be stopped in a level position (block the wheels if it is not perfectly level). Place the automatic transmission in neutral (manual transmissions can be put in reverse gear), turn the engine off, keep your foot off the service brake, and release the parking brake. NOTE: Be sure the vehicle does not roll. Looking at the air pressure gauge, the air loss should not exceed 2 pounds in one minute.

This test checks for leaks in the air lines. Continuing from the STATIC BRAKE TEST above, depress the service brake pedal and keep it fully depressed for one minute. Watch the air gauge needles. After the air pressure has stabilized (the needles stop moving), the air loss should not exceed 3 pounds in one minute (tap the gauge occasionally). One caution, do not apply too much pressure on the service brake pedal.

This test checks to see that the low air warning system is working. Continuing from the APPLIED BRAKE TEST above, turn ignition switch fully on but do NOT start the engine. Look at the air pressure gauge and continuously pump the service brake until you can see the warning light come on and/or hear the warning buzzer. If your buzzer sounds because of the low oil pressure switch, just look for the low air warning light to come on. The low air light should come on at pressures below 60 pounds. NOTE: If the low air warning light or buzzer comes on while you are driving, immediately move off the highway because soon after the low air warning comes on the emergency brake will be applied automatically and you will stop where ever you are at that time.

This test checks to see whether the parking brake applies automatically if the air pressure gets too low. Continuing from the LOW AIR WARNING DEVICE TEST above, turn off the ignition and continue to pump the service brake pedal until the parking brake handle pops up of its own accord. This should happen at pressures below 45 pounds. After it pops up, start the engine but do NOT touch the parking brake (it is on at this point). Try to SLOWLY drive the vehicle with the low air pressure warning on. The emergency brake is still on so it should hold you back.

This test checks whether the emergency brake works manually. Continuing from the EMERGENCY BRAKE SYSTEM TEST, keep the engine running and allow the air pressure to build up until it is at full pressure (the spitter valve will pop). Put the coach in Drive and release the parking brake. Drive the coach at about 5 miles per hour and apply the parking brake (not the service brake pedal). It should stop you fairly quickly.

This test checks whether the service brakes are working and also whether they may need adjusting. Continuing from the EMERGENCY BRAKE TEST, drive the coach at about 5 miles per hour, loosely hold the steering wheel and apply the service brake pedal firmly to see whether the steering wheel pulls to the left or the right. If it pulls in either direction you may need to have your brakes adjusted. Furthermore, if you apply the service brake and the two needles on the air gauge move farther apart than their normal (small) divergence, this is an indicator that the brakes may need adjusting.