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, release the brakes. Brake again as soon as the tires gain traction.

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.

Trailer Air Lines

Every combination vehicle has two air lines. These lines are the service line and the emergency line. They run between each vehicle, such as tractor to trailer, trailer to dolly, and dolly to second trailer.

SERVICE AIR LINES.- The service line carries air that is controlled by the foot brake or the trailer hand brake. Depending on how hard the foot brake is engaged, the pressure in the service line will similarly change. The service line is connected to relay valve(s) on the trailer to apply more or less pressure to the trailer brakes. As pressure increases in the service line, the relay valve opens and sends air pressure from the trailer air tanks to the trailer brake chambers, thus applying the trailer brakes.

EMERGENCY AIR LINES.- The emergency line has two purposes. First, it supplies air to the trailer air tanks. Second, the emergency line controls the emergency brakes on the combination vehicle. Loss of air pressure in the emergency line causes the trailer emergency brakes to activate. The pressure loss could be caused by a trailer breaking loose and tearing apart the emergency air hose. The loss could also be the result of a hose, metal tubing, or other parts breaking and causing an air leak. When the emergency line loses pressure, it also causes the tractor protection valve to close, causing the air supply knob to pop out.

Emergency lines such as hoses couplers, and other parts, have a red covering. The red covering allows you to separate the emergency lines from the service lines which have a blue covering.

Hose Couplers

Hose couplers, commonly known as glad hands (fig. 7-11), are coupling devices used to connect the service and emergency air lines from the truck or tractor to the trailer. The glad hands have rubber seals, known as rubber grommets, that prevent the air from escaping. Clean the rubber grommets before you connect the glad hands. When connecting the glad hands, press the two seals together with the glad hands at a 90-degree angle to each other. A turn of the glad hands attached to the hose joins and locks the couplers.

Some vehicles have "dead end" or dummy glad hands to which the hoses should be connected when not in use. This prevents water and dirt from getting into the glad hands and the air lines. This is very important because keeping the air system clean is a critical factor.

Figure 7-11.-Glad hands.

When connecting the glad hands, ensure the proper glad hands are coupled together. On some equipment, metal tags are attached to the lines with the words service and emergency stamped on them. The color blue is used for the service line and the color red for the emergency line connections.

If the air lines are crossed, supply air is sent to the service line instead of going to charge the trailer air tanks; therefore, air is not available to release the trailer spring brakes (parking brakes). If the spring brakes do not release when you push the trailer air supply control knob, check the air line connections.

CAUTION

Older trailers do not have spring brakes. If the air supply in the trailer air tanks has leaked out, emergency brakes will not exist, and the trailer wheels will turn freely. If you cross the air lines, the trailer will roll; however, there will be no trailer brakes.

NOTE: Always test the trailer brakes before driving by engaging the hand valve or by pulling the tractor protection valve. Once these brakes are engaged, shift the tractor to low gear and pull gently against the brake system to make sure the brakes work.