Tractor Air Brake System Explained

This topic will explain the function of each brake system component used on this standard tractor air brake system. The trailer air brake system is explained in the trailer air brake system topic. For detailed information about foundation brakes and air brake mechanical linkage, refer to our mechanical linkage topic. For more detailed information about air brake pressure controls, refer to our brake pressure components topic. The following list of topics are contained in this description. A typical tractor brake diagram is listed below, so lets get started.

Compressor and Reservoirs

In the drawing above, the compressor (bottom left) builds up air pressure as it passes through the air-dryer on to the supply reservoir. The D-2 governor on the compressor keeps the tractor air supply pressure between 100 and 120 PSI. The safety valve on the supply reservoir prevents the air pressure from exceeding 150 PSI. The supply reservoir feeds the other two tanks via a check valve. The check valve prevents the front axle service reservoir and the rear axle service reservoir from bleeding off pressure back to the other tank or into the supply reservoir if one of these tanks should suddenly loose air pressure.

Double-check Valves

Notice the double check valve (dc-4) and the double check and brake light switch valve (ds-2). Both of these valve take supply air pressure from two different sources and share their use. If one of the supplies drops pressure, then that supply is isolated from the other supply, and only the other supply provides air pressure to the shared port. The brake light switch activates with pressure from either source.

Brake Pedal Valve

The brake pedal value is two independent service brake valves in one. The top half provides service brake air pressure to the front axle. The lower half provides service brake air pressure to the rear axles. Both halves regulate the air pressure (PSI) to be relative to pedal movement. More pedal movement results in more air pressure to the associated axle(s).

Trailer Control Valve

The trailer control valve allows independent service brake application to the trailer only. This valve also provides air pressure relative to the handle movement. More handle movement, more air pressure. Never ever use this valve as a parking brake since loss of service brake air pressure would allow the rig to roll. Always use the spring brake valves for parking.

Quick Release, Relay and Anti-Compounding Valves

Notice the quick release valve (qr-1) between the two front brake chambers. This valve is mounted close to the brake chambers and allows rapid exhaust of the brake chambers when the front brake chamber pressure is released. When you let up on the brake pedal, this valve will vent the front brake air chambers to rapidly follow the brake pedal pressure. For partial release of the brake pedal, the quick release valve will rapidly exhaust the pressure down to, but not below, the current brake pedal pressure. In other words, the front brake chambers will rapidly follow your brake pedal pressures.

A relay valve (r-14) at the rear axles provides three functions. It acts like a quick release valve when you apply the parking brakes (exhausts the spring brake chambers). It acts like a relay valve which quickly pressurizes the spring brake chambers to release the spring brakes when the parking brakes are removed. Relay valve (r-14) also provides an anti-compounding function.

Anti-compounding is a method to prevent simultaneous application of the service brake and the parking brake (spring brake) at the same time. For example, a truck is stopped on a hill, and while the driver holds the foot brake, the driver also sets the parking brakes. Setting the parking brakes releases the air from the spring brake chambers (see air brake chambers) which allows the powerful spring inside the spring brake chamber to push the slack adjuster which sets the spring brakes. This all happens while the service brake chamber is already pushing on the slack adjuster to set the service brake. This combined force of spring brake and service brake force is additive and puts excessive force on the slack adjuster (the sum of both forces), which can lead to premature failure of the slack adjuster or over tightening of automatic slack adjusters.

Relay valve (r-14) has a double check valve built into it, which serves as the anti-compounding function. One port of the double check valve is connected to the parking brake valve, and the other port of the double check valve is connected to the rear axle service brake lines. Pressure from either source will release the service brake chambers. Therefore, in the above situation, the driver holding the service brakes on, places pressure into the spring brakes which releases the spring brakes while the service brake pedal is pressed. When the driver sets the parking brakes, relay valve (r-14) still holds pressure in the spring brake chambers and that supply is from the foot brake pedal.

When the driver lets up on the foot brake pedal, relay valve (r-14) quick releases the spring brake chambers, and the spring inside the spring brake chambers continues to hold the parking brake on. If the driver were to step on the foot pedal after the parking brakes were on, the service brake pressure would go to the service brake chambers and also go to the spring brake chambers. As the force and pressure increases in the service brake chamber to push on the push rod, the same pressure releases the spring brake chamber spring, which reduces the force placed on the push rod by the spring. This is anti-compounding, when the force from the spring brake chamber is reduced to compensate for the force applied by the service brake chamber.

Bobtail Proportioning Valves

While we are at the front brake system, let's take a look at the bobtail ratio valve (lq-5). This valve automatically reduces the front axle brake chamber pressures below the brake pedal pressures while a trailer supply exists. This helps in preventing jackknifes from slippery road conditions. With no trailer supply pressure, the front axle brake chambers receive 100% of the brake pedal pressure for increased braking while bob-tailing.

At the rear wheels we need to take a look at the bobtail proportioning relay valve (bp r-1). This valve serves two purposes. First it is a relay valve, which means that the rear axle brake pressure signal causes this valve to quickly apply and release the rear axle service brakes in response to the brake pedal pressures. It's second purpose is to reduce the rear axle brake chamber pressures while the tractor is bobtailing since bobtailing reduces the weight on the rear axle tires and results in easier skidding of these tires. The combined use of this valve and the bobtail ratio valve described above, applies more braking power to the front axle and less braking power to the rear axle while bobtailing. While not bobtailing, these valves apply more braking power to the rear axles and less braking power to the front axle to reduce jackknife tendencies. Both valves sense the trailer supply line pressure.

Cab Dash Control Valves

The tractor control valve (tp-3) is normally used to shut off the air supply to the trailer during trailer disconnect. The valve body is mounted at the rear of the tractor cab, and is controlled by a dash mounted valve (dash mounted valve is not shown in this drawing). This valve can also be used to manually isolate the trailer air lines during an emergency trailer breakaway or during a severe trailer service line air leak. This valve also monitors the trailer supply line, and if it senses trailer supply line pressure less than approximately 40 PSI, it activates the valve to shut off both the trailer service air line and the trailer supply air line. This has two effects. It protects the tractor air supply from further loss, and it allows the trailer air leak to set the trailer spring brakes.

At the top left corner of the diagram, you will find an MV-3 Control Module. This module can be used in lieu of the system park and trailer park valves shown in the diagram. The 4-sided dash valve button labeled system park (pp-1) is used for parking brakes. In the out position, it exhausts the spring brake pressure signal to the tractor spring brake relay valve (r-14) which in turn exhausts the tractor spring brake air chambers. This forces the spring brakes to set on the tractor. This valve is pressure sensitive (and can be manually overridden) and will pop out when it sees a low supply pressure in the air reservoirs. Normally this valve would be pushed in when the truck air reservoirs have exceeded 90 PSI, and you are ready to start driving the truck.

The 6-sided valve button labeled trailer park (pp-7) is usually identified as the trailer air supply valve. It has an air operated interlock in the lower body which will apply the trailer spring brakes (exhaust the air pressure) whenever the tractor spring brakes are applied (loss of tractor spring brake signal pressure). This interlock ensures that the trailer spring brakes will always be applied along with the tractor spring brakes. If system supply pressure falls below 40 PSI, then this valve will automatically pop out and exhaust the trailer supply line which will set the trailer spring brakes. Normally this button is pushed in to release the trailer spring brakes.

Now things get a little more complicated. Notice the dash mounted (round button) labeled trailer release valve (PP-1). This valve is also pressure sensitive and will stay in manual positions as long as the minimum supply pressure is present. This valve is normally left in the out position. It will be forced in the out position the last time the supply air pressure dropped below its sensitive limit (the last time the tractor was shut down for an extended period of time). While in the out position, it ports the system park spring brake signal (tractor spring brake signal) over to the trailer park valve's (PP-7) air operated interlock. Any time that the trailer release valve (PP-1) is out, then application of the system park valve will exhaust the spring brake pressure signal, and through the trailer park (PP-7) valve air interlock section, force the trailer park valve to exhaust the trailer supply line pressure. This will force the trailer spring brake valve (not shown) to exhaust the trailer spring brake chambers, thereby setting the trailer spring brakes just because the tractor spring brakes were set. If you fully understand this paragraph, then you are a good man Charlie Brown! You may have to reread this paragraph a few times.

The above paragraph is what the trailer release valve (PP-1) does while it is not in use. Its primary purpose when it is pushed in, is to isolate the trailer spring brakes application from the tractor spring brakes application. With this valve in, the trailer spring brakes will not be set when the tractor spring brakes are set. Now the trailer park valve doesn't know when the tractor spring brake signal pressure is missing, and it does not force automatic application of the trailer spring brakes.

This valve is used to release the trailer spring brakes while the tractor spring brake valve (sr-1) is modulating the tractor spring brakes signal pressure, using the front tractor brake pressure signal. This situation arises when the rear axle reservoir has lost air pressure. Another tough paragraph Charlie Brown! The next paragraph will explain the action of the tractor spring brake valve (SR-1) in more detail.

Spring Brake Valve

The spring brake valve (SR-1) serves two functions. During normal operation it limits hold-off pressure (spring brake air pressure signal) to the tractor spring brakes relay valve (R-14) to 90 or 95 PSI. The other function is to use the spring brakes for stopping power when the rear axle brake air supply fails, while the front axle brakes still work. This is a pretty revolutionary concept. If the rear axle service brake pressure fails, just make the rear axle spring brakes apply stopping power which follows the front axle brake application.

If a loss of pressure occurs in the rear axle service brake supply, (SR-1) will then provide a modulated spring brake application pressure to the spring brake relay valve (r-14) which is inversely proportional to the front axle service brake pressure. As the front axle brake pressure increases, the spring brake air pressure decreases. This allows application of the spring brakes to provide rear axle braking (using the spring brake spring force) which follows the front axle braking force. As the pressure in the front axle brake circuit goes up, the spring brake chamber pressure is bled down, which gives rear axle braking force from the spring chambers.

When the front axle brakes are released, then the pressure to the spring brake chamber is increased, which releases the spring brake action at the rear axles.

Cab Valve Operations

Now that we have explained all the brake components on the diagram, lets take a moment and see how the driver would use the cab air brake controls. The trailer park valve (commonly referred to as the trailer air supply valve) is not normally used for parking because it only applies the trailer spring brakes. Instead the system park valve (commonly referred to as the parking brake) would be used because its operation results in both tractor and trailer spring brakes being applied.

Never use the trailer control valve as a parking break! If air pressure to this valve would fail, then the rig could roll if the spring brakes air pressure was still normal.

OEM Considerations

This diagram provided by Bendix demonstrates a typical tractor air brake system without anti-lock brakes and without traction control features. Many of the newer brake valves provide multiple functions within the same valve body. Now that you have a typical air brake system understanding, you should consult the air brake diagrams for the particular tractor manufacturer. That diagram should now make a lot more sense. If you see new types of valves which were not described on this diagram, refer to our detailed valve description for information about other air brake valve components that Bendix manufacturers.

Return to top of page , Driver Knowledge Page , Truck Brakes page , HomePage.