125 pounds per square inch (psi), the governor stops the compressor from pumping air. When the tank pressure falls to the cut-in pressure at about 100 psi, the governor allows  the  compressor  to  start  pumping  again. Air Storage Tanks Air  storage  tanks  (reservoirs)  are  used  to  hold compressed air. The number and size of air tanks varies among vehicles. The tanks hold enough air to allow the brakes to be used several times, even if the compressor stops  working. NOTE: Compressed air usually has some water and some compressor oil in it which is bad for the air brake system.  For  example,  the  water  can  freeze  in  cold weather and cause brake failure. The water and oil tend to collect in the bottom of the air tank; therefore, each air tank is equipped with a drain valve in the bottom. The two types of drain valves areas follows: 1.  The  manual  valve  shown  in  figure  3-45  is operated by turning it a quarter turn or by pulling a cable. Part of your post-operational procedures is to drain all air tanks at the end of each day. 2. The automatic valve automatically expels the water and oil. This system may also be equipped with a manual  drain. Alcohol Evaporator Some air brake systems have an alcohol evaporator to put alcohol into the air system. This helps reduce the risk of ice in air brake valves and other parts during cold weather. Ice inside a brake system can make the brakes stop  working. Figure 3-45.-Air tank manual drain valve. If your vehicle has an alcohol system, the container should be checked during each prestart operation and filled up as necessary. Daily draining of the air tanks is still required to get rid of the water and oil. Safety Valve A safety relief valve is installed in the first tank into which the air compressor pumps air. 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 has to release air pressure, something is wrong in the air brake system. This should be documented to inform the mechanic inspectors. Brake Pedal The brakes are applied by depressing the brake pedal  (also  called  the  foot  valve,  or  treadle  valve) that  gives  the  operator  control  of  the  air  brake system. When the brake pedal is engaged, air from the  air  tanks  flows  through  the  brake  pedal  valve through the brake lines to the brake chambers close to the wheel brakes that contain flexible diaphragms. The force of the air admitted into these chambers causes the diaphragms  to  operate  the  brake  shoes  through  a mechanical   linkage. 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 allows some compressed air out of the system; therefore, the air pressure in the tanks is reduced and it must be recharged by the air compressor. Pressing and releasing the pedal unnecessarily may release air out faster than the compressor can replace it, and should the pressure become  too  low,  the  brakes  cannot  work  properly  and brake failure will occur. Pressure Gauge An air pressure gauge lets you know if you have proper air pressure within the reservoir. A low air warning  device  should  cut  on  before  the  pressure drops to less than 60 psi in the air tank. This gauge is usually on the instrument panel of a truck or bus. If the pressure fails to buildup or exceeds the maximum limits after building up, secure the truck until the fault is corrected. Hand Brake Valve Independent  control  of  brakes  is  necessary  under bad conditions, especially if you have to put on the 3-25