5.4.4 – Stopping Distance  5.4.5 – Brake Fading or Failure  5.4.7 – Low Air Pressure  5.4.6 – Proper Braking Technique  Stopping distance was described in Section 2  under "Speed and Stopping Distance." With air  brakes there is an added delay - “Brake Lag”. This  is the time required for the brakes to work after the  brake pedal is pushed. With hydraulic brakes (used  on cars and light/medium trucks), the brakes work  instantly. However, with air brakes, it takes a little  time (one half second or more) for the air to flow  through the lines to the brakes. Thus, the total  stopping distance for vehicles with air brake  systems is made up of four different factors.  Perception Distance + Reaction Distance + Brake  Lag Distance + Braking Distance = Total Stopping  Distance  The air brake lag distance at 55 mph on dry  pavement adds about 32 feet. So at 55 mph for an  average driver under good traction and brake  conditions, the total stopping distance is over 450  feet. See Figure 5.6.  Brakes are designed so brake shoes or pads rub  against the brake drum or disks to slow the vehicle.  Braking creates heat, but brakes are designed to  take a lot of heat. However, brakes can fade or fail  from excessive heat caused by using them too  much and not relying on the engine braking effect.  Excessive use of the service brakes results in  overheating and leads to brake fade. Brake fade  results from excessive heat causing chemical  changes in the brake lining, which reduce friction,  and also causing expansion of the brake drums.  As the overheated drums expand, the brake shoes  Section 5 – Air Brakes  If the low air pressure warning comes on, stop and  safely park your vehicle as soon as possible. There  might be an air leak in the system. Controlled  braking is possible only while enough air remains  in the air tanks. The spring brakes will come on  when the air pressure drops into the range of 20 to  45 psi. A heavily loaded vehicle will take a long  distance to stop because the spring brakes do not  work on all axles. Lightly loaded vehicles or  vehicles on slippery roads may skid out of control  when the spring brakes come on. It is much safer  to stop while there is enough air in the tanks to use  the foot brakes.  For example, if your "safe" speed is 40 mph, you  would not apply the brakes until your speed  reaches 40 mph. You now apply the brakes hard  enough to gradually reduce your speed to 35 mph  and then release the brakes. Repeat this as often  as necessary until you have reached the end of the  downgrade.  Remember. The use of brakes on a long and/or  steep downgrade is only a supplement to the  braking effect of the engine. Once the vehicle is in  the proper low gear, the following is the proper  braking technique:  and linings have to move farther to contact the  drums, and the force of this contact is reduced.  Continued overuse may increase brake fade until  the vehicle cannot be slowed down or stopped.  Brake fade is also affected by adjustment. To  safely control a vehicle, every brake must do its  share of the work. Brakes out of adjustment will  stop doing their share before those that are in  adjustment. The other brakes can then overheat  and fade, and there will not be enough braking  available to control the vehicle(s). Brakes can get  out of adjustment quickly, especially when they are  hot. Therefore, check brake adjustment often.  2005 Commercial Driver’s License Manual  • Apply the brakes just hard enough to feel a  definite slowdown.  • When your speed has been reduced to  approximately five mph below your "safe" speed,  release the brakes. (This application should last  for about three seconds.)  • When your speed has increased to your "safe"  speed, repeat steps 1 and 2.  Page 5-9  Figure 5.6 