Air Brake System Explained
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.
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
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.
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).
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.
Release, Relay and Anti-Compounding Valves
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
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
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.
(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.
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.
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.
Dash Control Valves
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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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
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