Spring  Brakes release the brakes. Brake again as soon as the tires gain traction. All trucks, tractor-trailers, and buses are equipped with emergency and parking brakes. These brakes are applied  by  the  mechanical  force  produced  by  a  spring brake. When you are driving, the powerful springs are held back by air pressure. A leak in the air pressure system  causes  the  springs  to  engage  the  brakes. Engaging  the  parking  brake  control  on  the  dashboard releases the air holding back the springs. This action allows the springs to engage the brakes. Tractor and straight truck spring brakes engage when  the  air  pressure  drops  to  a  pressure  ranging between 20 to 45 psi. Do not wait for the brakes to engage  automatically.  When  the  low  air  pressure warning light and buzzer activates, you should bring the vehicle to a safe stop immediately, while you can still control the brakes. The braking power of the spring brakes depends on the adjustment of the brakes. If the brake adjustment is incorrect,  the  regular,  emergency,  and  parking  brakes cannot work correctly. Parking  Brake  Controls On  newer  vehicles,  a  diamond  shaped,  yellow, push-pull  control  knob  is  used  to  engage  or  disengage the  parking  brakes.  When  the  knob  is  pulled  out,  the brakes are engaged. By pushing the knob in, you can release the brakes. On older vehicles, the parking brake may be controlled by a lever. You should engage the parking brake anytime the vehicle is parked. Using Air Brakes When using air brakes, you should push the brake pedal down and control the pressure so the vehicle comes to a smooth, safe stop. If your vehicle is equipped with a manual transmission, do not push the clutch in until the engine rpm is lowered to idle speed. When stopped, select a starting gear. EMERGENCY  STOPS.—   When  applying  air brakes in an emergency stop, brake so you can steer and keep your vehicle in a straight line. Use methods of controlled  or  stab  braking. Controlled  Braking.—  Controlled braking is also called   “squeeze”   braking.   Controlled   braking   is applying the brakes as hard as possible without locking the wheels. Do not turn the steering wheel while doing this. If steering is required or the wheels begin to slide, Stab Braking.—The  stab  braking  method  requires applying the brakes as hard as possible and releasing them when the wheels lock up. As soon as the wheels start to roll, apply the brakes fully again. It can take up to 1 second for the wheels to start rolling after releasing the brakes. You should stay off the brakes long enough to  get  the  wheels  rolling  again;  otherwise,  the  vehicle may not stay in a straight line. DOWNHILL BRAKING.— The correct method for going down long grades is to use a low gear and travel at a slow speed that allows a fairly light, steady use of the brakes to prevent the vehicle from speeding up. When you go slow and apply light pressure, the brakes cool down and work properly. CAUTION When going down a hill, overuse of the brakes can make them get too hot which causes the brakes to fade. This can cause the operator to press down harder on the pedal to gain the required  braking  power.  Prolonged  action  like this can cause the brakes to fade until they stop working. Trailer Hand Valve The trailer hand valve (also called the trolley valve) controls the trailer brakes. Do not use it when driving because of the danger of making the trailer skid. The foot brake sends air to all of the brakes on the vehicle, including the trailer. There is much less danger of causing a skid or jackknife when only the foot brake is used. Tractor Protection Valve The tractor protection valve keeps air in the tractor or truck should the trailer break away or develop a bad air leak. The valve is controlled by the  trailer air supply control valve in the cab of the tractor or truck. The control valve allows the opening and shutting of the tractor  protection  valve.  The  tractor  protection  valve closes  automatically  when  the  air  pressure  drops  to  a range of 20 to 45 psi. When the tractor protection valve closes, it stops any air from escaping from the tractor. It also shuts off the air from the trailer emergency line, causing the trailer emergency brakes to engage. 7-10