mtrumpet wrote:
If you're unfamiliar with the Air Brake Test, here it is. This is the standard proceedure. (This was posted by foosh1 here on the forum back on 3/2/10)
1) Start vehicle and let pressure build up to proper level (usually 120 psi)
2) Leave parking brake engaged, put trannie in D and gently press the accelerator. It's called a tug test, and parking brake should hold the vehicle.
3) Stop engine, but leave key in whatever position that allows you to read your air pressure gauges, chock vehicle if necessary and release parking brake.
4) Press brake pedal very hard and start timing one minute. Watch gauges and check to see you don't lose more than 3 PSI in one minute (4 psi for a combo vehicle). Listen for leaks.
5) Start pumping the brake pedal rapidly, and you'll hear air bleeding off. When PSI drops below a certain level a warning buzzer should sound, and when it drops a little lower the parking brake will automatically pop out. (This is a safety feature that allows you to stop if you were to lose your service brakes).
6) You're done.
NOTE! It is very important that you NEVER press on the brake pedal hard while the parking brakes are engaged. You could damage the springs that operate the parking brake system!!
The last part of this post isn't entirely true. Modern air brake systems employ an anti-compounding valve to prevent excessive air pressure in the brake chambers.
Anti-Compounding valve is a relay valve with a double-check valve built into it
To see how anti-compounding works, let's use the example where a truck is stopped on a hill, and while the driver holds the foot brake, the driver also sets the parking brakes. Setting the parking brakes would normally release the air from the spring brake chambers which would allow the powerful spring inside the spring brake chamber to push the slack adjuster which sets the spring brakes. This all happens while the service brake chamber is already pushing on the slack adjuster to set the service brake. This combined force of spring brake and service brake force is additive and puts excessive force on the slack adjuster (the sum of both forces), which can lead to premature failure of the slack adjusters, or mistaken automatic slack adjuster over tightening.
One supply port of the anti-compounding valve is connected to the parking brake valve, and the other supply port of the anti-compounding valve is connected to the rear axle service brake pressure signal. Pressure from either source will activate the spring brake relay valve and fill the spring brake chambers, thereby releasing the spring brake. Therefore, in the above situation, the driver holding the service brakes on the hill, places service brake air pressure into the service brake chambers which set the service brakes. When the driver now sets the parking brakes, the anti-compounding valve still holds pressure in the spring brake chambers, from the foot brake pedal service brake supply port; even while the parking brake valve supply port has purged all air pressure to the parking anti-compounding supply port. This air pressure into the spring brake chamber, holds off the spring force from the spring brake chamber as long as the service brakes are applied, and only the service brake air chamber force is applied to the slack adjuster. This is anti-compounding, when the spring force is delayed while the service brake force exists.
After setting the parking brakes, and when the driver then lets up on the foot brake pedal, the service brake supply port of the anti-compounding valve looses air pressure, and the parking brake supply port still has no air pressure. Since the spring brake relay valve now sees no signal air pressure from either supply port, it quickly vents the spring brake chambers which set the spring brake force on the slack adjuster. So the service brakes stopped holding force on the slack adjuster at the same time that the spring brakes commenced to hold force on the slack adjuster. This is continued anti-compounding control of the brakes.
If the driver were to later step on the foot pedal after the parking brakes were on, the service brake pressure signal would go to the service brake supply port of the anti-compounding valve, and fill the spring brake chambers to release the spring brake force, at the same time as the service brake air chamber applies more brake force on the slack adjuster. So now, as the service brake force increases, the spring brake force decreases. This is also anti-compounding, when the force from the spring brake chamber is reduced to compensate for the additional force being applied by the service brake chamber.
