2005 Commercial Driver’s License Manual  Section 5 – Air Brakes  Page 5-3  These gauges tell you how much pressure is in the  air tanks.  the control is in the "normal" position to have  normal stopping power.  Many vehicles have automatic front wheel limiting  valves. They reduce the air to the front brakes  except when the brakes are put on very hard (60  psi or more application pressure). These valves  cannot be controlled by the driver.  This gauge shows how much air pressure you are  applying to the brakes. (This gauge is not on all  vehicles.) Increasing application pressure to hold  the same speed means the brakes are fading. You  should slow down and use a lower gear. The need  for increased pressure can also be caused by  brakes out of adjustment, air leaks, or mechanical  problems.  A low air pressure warning signal is required on  vehicles with air brakes. A warning signal you can  see must come on before the air pressure in the  tanks falls below 60 psi. (Or one half the  compressor governor cutout pressure on older  vehicles.) The warning is usually a red light. A  buzzer may also come on.  Another type of warning is the "wig wag." This  device drops a mechanical arm into your view  when the pressure in the system drops below 60  psi. An automatic wig wag will rise out of your view  when the pressure in the system goes above 60  psi. The manual reset type must be placed in the  "out of view" position manually. It will not stay in  place until the pressure in the system is above 60  psi.  All trucks, truck tractors, and buses must be  equipped with emergency brakes and parking  brakes. They must be held on by mechanical force  (because air pressure can eventually leak away).  Spring brakes are usually used to meet these  needs. When driving, powerful springs are held  back by air pressure. If the air pressure is  removed, the springs put on the brakes. A parking  brake control in the cab allows the driver to let the  air out of the spring brakes. This lets the springs  put the brakes on. A leak in the air brake system,  which causes all the air to be lost, will also cause  the springs to put on the brakes.  Tractor and straight truck spring brakes will come  fully on when air pressure drops to a range of 20 to  45 psi (typically 20 to 30 psi). Do not wait for the  brakes to come on automatically. When the low air  pressure warning light and buzzer first come on,  bring the vehicle to a safe stop right away, while  you can still control the brakes.  The braking power of spring brakes depends on  the brakes being in adjustment. If the brakes are  not adjusted properly, neither the regular brakes  nor the emergency/parking brakes will work right.  On large buses it is common for the low pressure  warning devices to signal at 80-85 psi.  Drivers behind you must be warned when you put  your brakes on. The air brake system does this  with an electric switch that works by air pressure.  The switch turns on the brake lights when you put  on the air brakes.  Some older vehicles (made before 1975) have a  front brake limiting valve and a control in the cab.  The control is usually marked "normal" and  "slippery." When you put the control in the  "slippery" position, the limiting valve cuts the  "normal" air pressure to the front brakes by half.  Limiting valves were used to reduce the chance of  the front wheels skidding on slippery surfaces.  However, they actually reduce the stopping power  of the vehicle. Front wheel braking is good under  all conditions. Tests have shown front wheel skids  from braking are not likely even on ice. Make sure  In newer vehicles with air brakes, you put on the  parking brakes using a diamond-shaped, yellow,  push-pull control knob. You pull the knob out to put  the parking brakes (spring brakes) on, and push it  in to release them. On older vehicles, the parking  brakes may be controlled by a lever. Use the  parking brakes whenever you park.  Caution. Never push the brake pedal down when  the spring brakes are on. If you do, the brakes  could be damaged by the combined forces of the  springs and the air pressure. Many brake systems  are designed so this will not happen. But not all  systems are set up that way, and those that are  may not always work. It is much better to develop  the habit of not pushing the brake pedal down  when the spring brakes are on.  5.1.13 – Front Brake Limiting Valve  5.1.12 – Stop Light Switch  5.1.15 – Parking Brake Controls  5.1.11 – Low Air Pressure Warning  5.1.10 – Application Pressure Gauge  5.1.14 – Spring Brakes 