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Air Brake Basics Part 3

In Cab Valves And Gauges

A system of high pressure tubing, hoses, fittings and valves provides a network of checks and balances which direct and regulate the compressed air to the various brake chambers which ultimately stop the vehicle.

It is the driver who directs the flow of air into the circuits of the air brake system by opening and closing various valves via the in-cab hand valves on the instrument panel and via the foot valve.

What automobile drivers consider the brake pedal is termed the foot valve, or treadle valve in air braked vehicles and is mounted either on the floor or fire wall of the cab. It truly is a valve in that the more it is pushed down, the more air it lets through. It operates both the tractor and trailer brakes when depressed. The treadle valve is shown in Figure 2-6. Its function is to regulate the air pressure in the service line (blue line) that controls how forcefully each brake is applied.

An optional application gauge on the instrument panel displays the amount of air pressure released into the braking system from the air tanks when the treadle valve is pushed. This gauge is visible in Figure 2-3. It shows the pressure actually used to apply the brakes and is directly proportional to how far the driver depresses the foot valve. A small amount of pressure on the brake pedal will cause as little as 5 psi to be applied to the system. If the pedal is pushed all the way to the floor, all available pressure in the reservoir tanks can be applied.

It is important to note that when a driver applies the brakes in a tractor, the only feedback he gets as to how hard he is pushing is through a return spring on the foot valve, which is not proportionate to the amount of brake application such as when a driver applies the brakes in an automobile. This lack of feedback forces the driver to be more aware of the capabilities of his equipment and the information provided by the indicators mounted on the dashboard and by the visual and kinesthetic feedback of how quickly his vehicle is slowing down.

The hand valve, as shown in Figure 2-7, is normally located on the steering column and its function is to apply the trailer brakes only, while the foot valve operates both the tractor and trailer brakes. The hand valve should be used minimally and only for certain  limited purposes, such as checking trailer brakes during trailer hookup or holding on a hill while the clutch is disengaged.

Some drivers use the hand valve for braking reasons that are unacceptable in professional driving including braking on a downhill, parking, or straightening a potential jackknife condition. Improper usage of the hand valve can potentially damage the equipment and prohibit its effective use when needed.

As previously discussed, the reservoir air pressure gauge is located on the dash and is extremely important for the driver to monitor while the truck is in motion. This gauge tells how much reserve air pressure remains in the braking system. A periodic inspection by the driver should confirm the presence of air pressure at a safe operational level, usually in the range of 110-120 psi.

If the pressure should fall to 60 psi, a failure in the system has occurred and a warning device will alert the driver to move the truck off the road and bring it to a stop as soon as possible. On most systems, a red light activates and a buzzer sounds. Both service reservoirs contain sending units so that if either air tank is low, the buzzer and light will sound.

The air pressure released by the foot and/or hand brake valve passes through the double check valve. Figure 2-2. Its primary function is to ensure that air passing through the hand valve goes only to the trailer brakes, and that air passing through the foot valve goes to the brakes on both tractor and trailer.

The passage of more than 5 psi of air through the double check valve activates the stop lamp switch which energizes the brake lights on the rear of the tractor and trailer. Pressure less than 5 psi does not activate the switch. Therefore, if a driver brakes gently and gradually the red brake lamps may not light.

The two other situations where brakes can be applied without the red brake lights showing are potentially more dangerous. If either of the two emergency brake systems are activated, the trailer brakes or tractor parking brakes are applied and the vehicle can come to a sudden stop with very little warning.

For safety and convenience functions, every air brake-equipped vehicle has two knobs on the dash board which operate control valves of the air brake system. One is the red, octagonal trailer air supply knob and the other is the yellow, diamond-shaped parking brake knob as shown in Figure 2-8.

When the red knob on the dashboard is pressed in, it operates an air valve which in turn operates the tractor protection valve. Once the tractor protection valve is opened, it maintains a pathway between the trailer supply tanks and the tractor air system.

Air pressure from the supply line flows to the spring brake chambers after the trailer reservoir tanks have reached 70 psi, compressing the springs and allowing the wheels to roll freely. When the air pressure in the tractor protection valve reaches 45 psi, the driver can then operate the treadle or hand valve to operate the trailer brakes. The driver may also apply the trailer spring brakes manually by pulling the red knob out.

Although the valve under the red knob is frequently referred to as the tractor protection valve, it is really only a control valve that can manually open or close the tractor protection valve which is located at the rear at the cab. The primary purpose of the tractor protection valve is to safeguard the tractor’s air supply from a serious leak or rupture in the trailer’s air system and was required as of 1955.


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