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The Daily Airbrake Routine - Using The Chart
The major concern with Air Brake Systems is that a lot can change in a short period of time - things over which you have no control from the driver's seat. Consequently, there are things which must be checked before commencing each trip - whether heading out for a long adventure, or just down to the truck wash! - and during the longer trips themselves. Let's look at them one at a time:
The Pre-Trip Check

1. Compressor Belt
The compressor is the heart of the airbrake system. No air, no brakes. Fortunately, you have the dash-mounted air gauge, and the low air warning system to give you some warning if the compressor fails, but it's best to be as sure as you can that it won't. If your compressor is belt-driven, the belt should be checked on a regular basis - before startup every morning. Check that the adjustment is correct - a loose belt will slip and eventually burn itself up - and that it is in good condition - no cracks, signs of burning, or deterioration of any kind. Be sure also that you are carrying a spare belt - of the correct size and quality - at all times. A blown belt will put you at the side of the road every bit as effectively as a blown tire.
2. Buildup Time
As the compressor gets on in life, its ability to produce air deteriorates. This doesn't happen all at once, but slowly. Consequently, it's a good idea to monitor this situation by regularly recording the time it takes - at warmup rpm - to reach a given pressure. 100 psi is a good number. This should be recorded before every trip, and should never reach more than 120 seconds.
3. Pressure Cutout
The compressor needs to be de-activated once a maximum pressure is reached. On most systems, this is 120 psi. At this point, the gauge should stop climbing, and you may even hear a change in sound from "back there" as the compressor cuts out. If the gauge continues to climb past the 120 mark, shut the engine down and investigate - the regulator, mounted on the compressor itself, would be the prime suspect. It can freeze in cold conditions (water condensation), or it may require the services of an air brake professional. NEVER attempt to drive with a stuck regulator, since the compressor has the power to do some serious damage to your system. There is a "last-resort" blow-off relief valve in the system, but what if it's stuck?
4. Pressure Cut-in
As the air in the reservoirs is used, the pressure drops until the regulator cuts the compressor back in. This must happen at the lower limit of your green zone on the air pressure gauge. To see that this will happen before you roll out, it's a good idea to "fan" the brake pedal after cut-out until the pressure drops to this point, then see that the gauge begins to climb as the regulator cuts back in.
5. Air Leak Check
Once you're satisfied that your compressor is acting properly, you should shut the engine down and do a "walk-around," (you'll be checking the slack adjusters at all four corners, anyway) listening for any sounds of leaking air. If you do hear any escaping air, track down the location, and make the necessary repairs. Remember, a small leak now can turn into a disastrously large one in a heartbeat!
6. Slack Adjuster Check
Finally, and this is the one that will change the most, the slack adjuster at each wheel must be checked and adjusted if necessary. The maximum permissible slack is one inch. If your slack at any wheel is reaching this maximum, you'll want to readjust the slack adjuster, and - this is important! - re-check the slack after adjustment! (It has been known for operators to turn the adjuster the wrong way, going to maximum slack and then tightening a half turn, leaving the brake completely non-functional!)
Slack adjustment may be checked by visual observation of the pushrod/slack adjuster assembly as an assistant applies brake from the driver's seat, or by manually pulling or pushing the end of the slack adjuster to the limit of its travel. A tool such as a Brake Buddy (TM) is useful for levering such movement, as well as for the actual adjusting. Nut size is usually 9/16".
NEVERdrive your vehicle - not even a few feet - with excessive brake slack!
A Brake Log
Finally, it is useful to maintain a log of your brake maintentance tasks. Review of the changes in numbers will provide an overview of the condition of the various elements of your brake system. For a printable brake log form, click here.