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Air Brake Basics Part 4

It is designed to automatically shut off the tractor’s air supply to the trailer whenever the pressure in the tractor air system is reduced to 40 ± 5 psi. In the event of a loss of tractor air pressure, the red valve on the dash pops out, closing off the air supply from the tractor to the trailer. This will simultaneously engage the trailer spring brakes and usually cause full wheel lock-up on the trailer.

For example, if the tractor-trailer combination turns a sharp corner and the supply air line connecting the tractor to the trailer catches on a projection on the deck plate and snaps, the air from the tractor’s dry tank supplying the trailer would exhaust until it dropped below 45 psi. The trailer air supply on the dash (red knob) would then pop out, disconnecting the supply air to the trailer. The tractor protection valve would then close, separating the tractor from the trailer service air system, and then the trailer brakes would lock, causing the entire vehicle to come to a stop.

Without these valves, the rupture would drain all of the air out of the tractor faster than the compressor could replace it. If the trailer broke away from the tractor, the trailer spring brakes would apply immediately and the tractor air brake system would retain enough air pressure for the driver to bring the tractor to a controlled stop.

The yellow knob mounted on the dashboard is labeled the parking brake valve and directly controls the application of the spring brakes on the tractor. When pushed in, the tractor spring brakes are released. When pulled out, the tractor spring brakes are activated which will simultaneously cause the trailer supply valve (red knob) to trip which will close the tractor protection valve and thus also activate the trailer spring brakes.

The parking brake valve is used only for applying the tractor’s spring brakes when the equipment has been parked. As a safety feature, if the air pressure in the tractor supply tank drops below 30 to 35 psi, the tractor parking brake valve closes. As described above, this will activate the brakes on both the tractor and the trailer.

In a car, when the brake pedal is depressed, the driver’s foot creates the pressure in the brake fluid which is transmitted to the brake cylinders at each wheel, where brake shoes are applied to the rotating brake drum in order to slow the car. When the brake pedal is released, pressure is removed from the brake system and the brakes on the wheels disengage.

This is not the case for an air brake system on a truck. In an air brake-equipped vehicle, the driver’s foot is not what produces pressure in the brake system. The driver’s foot merely opens a valve that admits the pressure. For pressure to be eliminated from the system and the brakes to be disengaged, the pressurized air is vented from the air lines by the quick release valves. These valves speed up brake release by providing vents through which pressurized air can escape quickly compared to traveling back to the tractor in order to be vented by the relay or foot valve.

Some tractors may have an optional bobtail proportioning relay valve which make the tractor brakes less sensitive when the tractor is not pulling a trailer (bobtailing).

Trailer Brakes

The compressor on the engine also supplies air to the trailer braking system. Because of the safety redundancy of having a dual air circuit system on the tractor, the circuit with the greatest air pressure will be the source of air for the trailer. This ensures that if there is a failure to either the front or rear axle air circuits on the tractor, the trailer brakes will still operate.

There are two air hoses from the rear of the tractor cab which connect to the front of the trailer and deliver pressurized air to the trailer braking system. These lines are referred to as the supply (or emergency) side and the service (or control) side of the trailer air system.

At the ends of both air lines shown in Figure 2-9, there are connectors, each containing a rubber grommet. These connectors are called glad hands and they match a like connector on the front face of the trailer as shown in Figure 2.10. Generally these lines are color coded where the service line is blue and the supply line is red. If the trailer connectors are not color coded, then both are usually black or silver and the emergency line will connect to the receptacle closest to the driver’s side on the front of the trailer. The connector toward the center of the trailer will be the service receptacle, unless the equipment is old or specially built.

If the lines are incorrectly switched, the pressure running from the emergency line into the service line receptacle activates the trailer brakes. This will prevent moving the vehicle until the mistake is corrected and can possibly cause damage due to brake compounding, which will be discussed later.

The two air lines going to the trailer serve two different functions and are not redundant. As is discussed later in this section, the four or more air brakes on each trailer require a significant volume of air to operate. If that quantity of air had to flow from the cab whenever the driver stepped on the treadle valve, an unacceptable delay time would exist before the trailer brakes would engage. The two air line trailer system shortens the delay time in the following manner.

The full air pressure of the tractor (normally 120 psi) is in the trailer supply line at all times. It is used to fill and keep filled the trailer air tank to maximum pressure. The service line only has air in it when the driver steps on the treadle valve or operates the hand valve.

The harder the driver steps on the treadle valve, the higher the air pressure developed in the service line. The advantage is that the service line does not need to move a large quantity of air when the treadle valve is activated.

When a small quantity of air enters the service line, it signals valves at the back of the trailer to open and allow the high pressure air in the trailer storage tank to activate the brakes. The delay time is on the order of 0.5 seconds.


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