Testing Air Brakes

This was from an article on air brakes in the September issue of the Family Motor Coaching magazine:

Let’s assume that a 200-horsepower engine is needed to accelerate a vehicle to 65 mph in one minute. For that vehicle to come to an emergency stop in 6 seconds (one-tenth of the time it took to reach 65 mph) requires 10 times the acceleration force, or 2,000 horsepower. Chassis builders take this into consideration when designing a braking system and determining a chassis’ gross vehicle weight rating (GVWR).

However, if the brake on just one wheel is out of adjustment or not operating, up to 25 percent of braking capability can be lost. Therefore, to ensure safe motorhome operation, the braking system should be tested regularly.

For someone whose driving style leans toward heavy engine brake usage and minimal service brake usage, chances are good the service brakes will not be in proper adjustment and won’t be very effective when they are needed most.

I tend to lean toward heavy engine brake usage and minimal service brake usage. Checking the air brakes is an important part of our Pre-trip Inspection.

This is my approach to testing our air brakes which is based on the air brake training that Lorraine and I did for our commercial driver’s license.

1. Make sure the coach is secure with the parking brake engaged

(In our training, we were told to also use a wheel chock. For our coach I do not chock any of the wheels)

2. Test the low air warning indicator

• Ensure air pressure is over 90 PSI, if not build air pressure
• Engine off, key on
• Pump the service brakes
• Warning at 65 PSI is good
• Warning below 55 PSI is defective

3. Test air pressure build

• Start Engine and run between 600-900 RPM
• Reduce air pressure to 90 PSI
• Time from 90 PSI to 120 PSI
• Less than 30 seconds is good
• More than 30 seconds vehicle is defective

4. Test air pressure governor settings

• Build air pressure to maximum and note cut-out pressure
• Cut-out must be between 120 PSI and 135 PSI
• 120 PSI is good
• More than 135 PSI is defective
• Pump down pressure 20 to 30 PSI until compressor cuts-in
• Cut-in should be at 100 PSI
• 100 PSI is good
• Less than 90 PSI is defective

5. Test air loss rate

• Release parking brake
• Apply service brake and shut engine off, key on
• Time for 1 minute and note any loss of air pressure
• 1 or 2 PSI loss is good
• More than 3 PSI loss in pressure is defective
• Re-engage parking brake

6. Test parking brake

• Ensure parking brake is engaged
• Start engine, select drive
• Attempt to move the coach forward
• Parking brake holds is good
• If coach moves than parking brake is defective

7. Drain air tanks

(Our coach is equipped with automatic moisture ejectors so I don’t do this step every time)

• Shut off engine, key on
• Drain front wet tank for 5-10 seconds
• Go back to gauges, see if any change to PSI
• Drain 2nd valve for 5-10 seconds and check for oil, sludge, water
• Drain 3rd valve for 5-10 seconds and check for oil, sludge, water
• If valves work – then coach is good
• If valves do not work then coach is defective