2005 Model Commercial Driver’s License Manual  The warning light and buzzer should come on  before the air pressure drops below 60 psi in either  system. If this happens while driving, you should  stop right away and safely park the vehicle. If one  air system is very low on pressure, either the front  or the rear brakes will not be operating fully. This  means it will take you longer to stop. Bring the  vehicle to a safe stop, and have the air brakes  system fixed.  You should use the basic seven-step inspection  procedure described in Section 2 to inspect your  vehicle. There are more things to inspect on a  vehicle with air brakes than one without them.  These things are discussed below, in the order  they fit into the seven-step method.  Check Air Compressor Drive Belt (if compressor is  belt-driven). If the air compressor is belt-driven,  check the condition and tightness of the belt. It  should be in good condition.  Check Slack Adjusters on S-cam Brakes. Park on  level ground and chock the wheels to prevent the  vehicle from moving. Release the parking brakes  so you can move the slack adjusters. Use gloves  and pull hard on each slack adjuster that you can  reach. If a slack adjuster moves more than about  one inch where the push rod attaches to it, it  probably needs adjustment. Adjust it or have it  adjusted. Vehicles with too much brake slack can  be very hard to stop. Out-of-adjustment brakes are  the most common problem found in roadside  inspections. Be safe. Check the slack adjusters. All vehicles built since 1994 have automatic slack  adjustors. Even though automatic slack adjustors  adjust themselves during full brake applications,  they must be checked.  Automatic adjusters should not have to be  manually adjusted except when performing  maintenance on the brakes and during installation  of the slack adjusters. In a vehicle equipped with  automatic adjusters, when the pushrod stroke  exceeds the legal brake adjustment limit, it is an  indication that a mechanical problem exists in the  adjuster itself, a problem with the related  foundation brake components, or that the adjuster  was improperly installed.  If the warning signal doesn't work, you could lose  air pressure and you would not know it. This could  cause sudden emergency braking in a single- circuit air system. In dual systems the stopping  distance will be increased. Only limited braking can  be done before the spring brakes come on.  Test Low Pressure Warning Signal.  Shut the  engine off when you have enough air pressure so  that the low pressure warning signal is not on. Turn  the electrical power on and step on and off the  brake pedal to reduce air tank pressure. The low  air pressure warning signal must come on before  the pressure drops to less than 60 psi in the air  tank (or tank with the lowest air pressure, in dual  air systems). See Figure 5.5.  Do the following checks instead of the hydraulic  brake check shown in Section 2, Step 7: Check  Brake System.  Check Brake Drums (or Discs), Linings, and  Hoses. Brake drums (or discs) must not have  cracks longer than one half the width of the friction  area. Linings (friction material) must not be loose  or soaked with oil or grease. They must not be  dangerously thin. Mechanical parts must be in  place, not broken or missing. Check the air hoses  connected to the brake chambers to make sure  they aren't cut or worn due to rubbing.  (Note: Automatic slack adjusters are made by  different manufacturers and do not all operate the  same. Therefore, the specific manufacturer’s  Service Manual should be consulted prior to  troubleshooting a brake adjustment problem.)  The manual adjustment of an automatic adjuster to  bring a brake pushrod stroke within legal limits is  generally masking a mechanical problem and is  not fixing it. Further, routine adjustment of most  automatic adjusters will likely result in premature  wear of the adjuster itself. It is recommended that  when brakes equipped with automatic adjusters  are found to be out of adjustment, the driver take  the vehicle to a repair facility as soon as possible  to have the problem corrected.  The manual adjustment of an automatic adjuster  should only be used as a temporary measure to  correct the adjustment in an emergency situation  as it is likely the brake will soon be back out of  adjustment since this procedure usually does not  fix the underlying adjustment problem.  Section 5 – Air Brakes  Page 5-6  5.3 – Inspecting Air Brake Systems  5.3.1 – During Step 2 Engine Compartment  Checks 5.3.2 – During Step 5 Walkaround  Inspection 5.3.3 – Step 7 Final Air Brake Check 