Spring Brakesrelease the brakes. Brake again as soon as the tires gaintraction.All trucks, tractor-trailers, and buses are equippedwith emergency and parking brakes. These brakes areapplied by the mechanical force produced by a springbrake. When you are driving, the powerful springs areheld back by air pressure. A leak in the air pressuresystem causes the springs to engage the brakes.Engaging the parking brake control on the dashboardreleases the air holding back the springs. This actionallows the springs to engage the brakes.Tractor and straight truck spring brakes engagewhen the air pressure drops to a pressure rangingbetween 20 to 45 psi. Do not wait for the brakes toengage automatically. When the low air pressurewarning light and buzzer activates, you should bring thevehicle to a safe stop immediately, while you can stillcontrol the brakes.The braking power of the spring brakes depends onthe adjustment of the brakes. If the brake adjustment isincorrect, the regular, emergency, and parking brakescannot work correctly.Parking Brake ControlsOn newer vehicles, a diamond shaped, yellow,push-pull control knob is used to engage or disengagethe parking brakes. When the knob is pulled out, thebrakes are engaged. By pushing the knob in, you canrelease the brakes. On older vehicles, the parking brakemay be controlled by a lever. You should engage theparking brake anytime the vehicle is parked.Using Air BrakesWhen using air brakes, you should push the brakepedal down and control the pressure so the vehiclecomes to a smooth, safe stop. If your vehicle is equippedwith a manual transmission, do not push the clutch inuntil the engine rpm is lowered to idle speed. Whenstopped, select a starting gear.EMERGENCY STOPS.— When applying airbrakes in an emergency stop, brake so you can steer andkeep your vehicle in a straight line. Use methods ofcontrolled or stab braking.Controlled Braking.— Controlled braking is alsocalled “squeeze” braking. Controlled braking isapplying the brakes as hard as possible without lockingthe wheels. Do not turn the steering wheel while doingthis. If steering is required or the wheels begin to slide,Stab Braking.—The stab braking method requiresapplying the brakes as hard as possible and releasingthem when the wheels lock up. As soon as the wheelsstart to roll, apply the brakes fully again. It can take upto 1 second for the wheels to start rolling after releasingthe brakes. You should stay off the brakes long enoughto get the wheels rolling again; otherwise, the vehiclemay not stay in a straight line.DOWNHILL BRAKING.— The correct methodfor going down long grades is to use a low gear andtravel at a slow speed that allows a fairly light, steadyuse of the brakes to prevent the vehicle from speedingup. When you go slow and apply light pressure, thebrakes cool down and work properly.CAUTIONWhen going down a hill, overuse of thebrakes can make them get too hot which causesthe brakes to fade. This can cause the operatorto press down harder on the pedal to gain therequired braking power. Prolonged action likethis can cause the brakes to fade until they stopworking.Trailer Hand ValveThe trailer hand valve (also called the trolley valve)controls the trailer brakes. Do not use it when drivingbecause of the danger of making the trailer skid. Thefoot brake sends air to all of the brakes on the vehicle,including the trailer. There is much less danger ofcausing a skid or jackknife when only the foot brake isused.Tractor Protection ValveThe tractor protection valve keeps air in the tractoror truck should the trailer break away or develop a badair leak. The valve is controlled by the trailer air supplycontrol valve in the cab of the tractor or truck. Thecontrol valve allows the opening and shutting of thetractor protection valve. The tractor protection valvecloses automatically when the air pressure drops to arange of 20 to 45 psi. When the tractor protection valvecloses, it stops any air from escaping from the tractor. Italso shuts off the air from the trailer emergency line,causing the trailer emergency brakes to engage.7-10
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