2005 Model Commercial Driver’s License Manual  5.1.5 – Alcohol Evaporator  5.1.6 – Safety Valve  5.1.7 – The Brake Pedal  5.1.8 – Foundation Brakes  5.1.9 – Supply Pressure Gauges  Section 5 – Air Brakes  Page 5-2  Foundation brakes are used at each wheel. The  most common type is the s-cam drum brake. The  parts of the brake are discussed below.  Brake Drums, Shoes, and Linings. Brake drums  are located on each end of the vehicle's axles. The  wheels are bolted to the drums. The braking  mechanism is inside the drum. To stop, the brake  shoes and linings are pushed against the inside of  the drum. This causes friction, which slows the  vehicle (and creates heat). The heat a drum can  take without damage depends on how hard and  how long the brakes are used. Too much heat can  make the brakes stop working.  S-cam Brakes. When you push the brake pedal,  air is let into each brake chamber. Air pressure  pushes the rod out, moving the slack adjuster, thus  All vehicles with air brakes have a pressure gauge  connected to the air tank. If the vehicle has a dual  air brake system, there will be a gauge for each  half of the system. (Or a single gauge with two  needles.) Dual systems will be discussed later.  Wedge Brakes. In this type of brake, the brake  chamber push rod pushes a wedge directly  between the ends of two brake shoes. This shoves  them apart and against the inside of the brake  drum. Wedge brakes may have a single brake  chamber, or two brake chambers, pushing wedges  in at both ends of the brake shoes. Wedge type  brakes may be self-adjusting or may require  manual adjustment.  Disc Brakes. In air-operated disc brakes, air  pressure acts on a brake chamber and slack  adjuster, like s-cam brakes. But instead of the s- cam, a "power screw" is used. The pressure of the  brake chamber on the slack adjuster turns the  power screw. The power screw clamps the disc or  rotor between the brake lining pads of a caliper,  similar to a large c-clamp.  Wedge brakes and disc brakes are less common  than s-cam brakes.  A safety relief valve is installed in the first tank the  air compressor pumps air to. The safety valve  protects the tank and the rest of the system from  too much pressure. The valve is usually set to  open at 150 psi. If the safety valve releases air,  something is wrong. Have the fault fixed by a  mechanic.  You put on the brakes by pushing down the brake  pedal. (It is also called the foot valve or treadle  valve.) Pushing the pedal down harder applies  more air pressure. Letting up on the brake pedal  reduces the air pressure and releases the brakes.  Releasing the brakes lets some compressed air go  out of the system, so the air pressure in the tanks  is reduced. It must be made up by the air  compressor. Pressing and releasing the pedal  unnecessarily can let air out faster than the  compressor can replace it. If the pressure gets too  low, the brakes won't work.  Some air brake systems have an alcohol  evaporator to put alcohol into the air system. This  helps to reduce the risk of ice in air brake valves  and other parts during cold weather. Ice inside the  system can make the brakes stop working.  Check the alcohol container and fill up as  necessary, every day during cold weather. Daily air  tank drainage is still needed to get rid of water and  oil. (Unless the system has automatic drain  valves.)  twisting the brake camshaft. This turns the s-cam  (so called because it is shaped like the letter "S").  The s-cam forces the brake shoes away from one  another and presses them against the inside of the  brake drum. When you release the brake pedal,  the s-cam rotates back and a spring pulls the brake  shoes away from the drum, letting the wheels roll  freely again. See Figure 5.2.  Figure 5.2 