How do you perform air brake test?

Chock the wheels or put the transmission in gear. Press in the brake control valve (the yellow knob), and, if applicable, the trailer supply valve (red knob). Let the air pressure settle, then press the brake pedal. You should not lose more than 3 psi/minute for a single vehicle, and 4 psi/minute for a combination vehicle. Any more than that, and you have a leak. Hold the brake for a minute, and check air pressure loss. After that, you have to drain air from the system - this is typically done by pumping the brakes repeatedly. Between 20 - 60 psi, the brake control valve (and trailer supply valve, if connected to a trailer) should pop out. All warning lights and chimes should be activated.

Once that's done, you make sure the vehicle is in neutral, and start the engine. Let the air build up. You should hear air release and notice a pause in the buildup of air between 115 - 130 psi.