The In-Cab Airbrake Test
The first thing to do is to break the test down into different categories. You can do this easily by using the acronym LAB (Leaks, Alarms, Buttons).
This test is usually done during the CDL pre-trip and is one of the last tests you will do before you go out on the road for the actual driving or skills test. Here's how you can prepare for this part of the test.
Chock the wheels - If you haven't done so already! Some states will require the truck to be chocked before you even start the test.
With the engine still running, check the air pressure. You are looking and listening for the air systems "governed cut-out pressure" to be reached. This happens when the air tank pressure reaches 100 psi to 125 psi. When the cut-out pressure is reached, you will hear the air compressor stop pumping air (sometimes hard to hear) or you will hear the air dryer exhaust air with a loud PSSSSHHHTTTT! Now you can start the air brake (LAB) test.
ALWAYS be sure to tell the CDL examiner what you're doing. He or she cannot read your mind.
Turn off the engine and turn the key back on. Why? Most trucks low air pressure warning buzzers will not work with the key off.
"L" - The first required task in the LAB test is to check for LEAKS.
Release all the brakes. Push in both brake buttons. The truck is chocked so don't worry about the truck rolling away. There will be an initial drop in air pressure. This is normal and it's basically the air leaving the storage tanks and going out to the rest of the system. After this initial drop, watch (and you could point to) the air supply gauge on the instrument panel and listen for any leaks.
To test for leaks push in and hold the foot brake pedal. Tell the examiner what you're doing and he/she will be probably be asking you what you're looking for. Explain to him/her that you are holding the brake pedal down for one minute and looking for an air loss of less than 4 psi in that minute (straight trucks require less than 3 psi).
Make sure you have some way of timing yourself and that you keep your foot on the brake for the whole minute. If the examiner sees you are doing things correctly, he/she may declare; "Okay, it's been a full minute," and move you to the next part of the test.
"A" - Now you will test for ALARMS.
Firmly and pretty quickly pump or "fan" the foot brake pedal and watch the gauge again. At around 60 psi the "low air" alarm or buzzer and/or the warning light should come on. If you forgot to turn the key on the alarms and warning lights will not work. You may fail the test right here or you may have to start the whole brake test over again.
"B" - The next task is to check the BUTTONS.
After acknowledging the alarms, continue to pump the brake pedal (slower but more deliberately) until both of the air supply buttons on the dash pop out. The buttons should pop out at about 30 psi.
After the buttons pop you will need to start the engine (ask the examiner if it's okay to continue) and build air pressure. While the air is building, ask the examiner if you can go pick up your wheel chock and do so. This is a great time to take a deep breath and try to gather your wits. Stow the wheel chock and climb back into the driver's seat. Put on your seatbelt and get ready to finish the brake test.
There are two final tests for the brake system
Parking Brakes: After you build the air pressure back to "normal" range, the brakes are still set from the air brake test so all you need to do is put the truck in a low gear and attempt to pull forward. Use light pressure. Do not release the clutch fully or you could damage the drive train. A slight "tug" against the brakes is what you want.
Service Brakes: To test the service brakes, release all the brakes (while covering the foot brake of course). Put the truck into low gear, check to see that the area ahead is clear and slowly get the truck moving forward. S-L-O-W-L-Y. After you get the truck moving, push in the clutch and apply the foot brake. The truck should stop without pulling to either side. Some states will require you to remove your hands from the wheel for this test.
That's it! The explanation's a bit wordy, but it really only takes about five minutes or so.
Written by: Fozzy