This document provides an overview of air brake systems that are commonly used on tractor-trailers and other large commercial vehicles. It describes the main components of air brake systems, including the foundation brakes, air supply subsystem, primary and secondary circuits, and spring brake subsystem. The air supply subsystem uses an air compressor, governor, air tanks, and optional air dryer to store and regulate compressed air. The foundation brakes use either drum brakes or disc brakes to stop the wheels. Air is routed through the primary and secondary circuits to apply the service brakes. The spring brake subsystem provides emergency braking ability.

1. Air Brakes
This presentation will cover the basics of air brakes that are commonly
found on tractor-trailers (Class 8 trucks).

2. Air Brakes
A moving vehicle is a form of energy. The energy comes from the
fuel that is burned by the vehicle’s engine. To stop a moving vehicle,
its energy must be changed into another form by the brake system.
Air brake systems are used on large commercial vehicles because
they can develop and transmit high mechanical forces over great
distances using simple components and connections. Air brake
systems use compressed air, which is stored in tanks, to produce the
force that applies the brakes at each wheel.

3. Air Brakes
Air brake design and performance are governed by safety
regulations in Canada and the United States.
These Standards are commonly referred to as Document #121 and
has been in effect since 1975.
In Canada the air brake systems must comply with the Motor
Vehicle Safety Act, and technical Standards Document # 121 of
North America.
Common 121 standards:
• Dash warning light off when air pressure reaches 60psi.
• Air build from 85-100psi within 40 seconds

4. Air Brakes
Components of Air Brakes:
 Foundation Brake Components:
 Drum brake system (S-Cam type most common)
 Disc brake system
 Air Supply subsystem
 Primary and Secondary subsystem
 Spring Brake (Parking and Emergency) subsystem

5. Air Brakes
Foundation Brake Components:
S-Cam Drum Brake
 Very common, e.g., found on most highway trucks/trailers
 Applied air to brake chamber, moves push rod to pivot slack
adjuster
 Slack adjuster “rotates” S-Cam shaft to apply brakes
 Brake shoe rollers “ride-up” S-Cam, spreading brake shoes
against brake drum.

6. Air Brakes
Foundation Brake Components:
S-Cam Drum Brake
 New brake chamber and brake linings
 You should always clean off (power wash) old brakes before
disassembly. Dust may contain harmful resins, etc.
Note: Asbestos is no longer used as a brake lining material.

7. Air Brakes
Foundation Brake Components:
S-Cam Drum Brake
 Cast alloy steels or fabricated steel. They have different C.O.F.
and should never be mixed.
 16.5 inch most popular.
 Organic brake linings are essentially composed of glue and a
strengthening material, this used to be asbestos but today
different materials are used because asbestos is a health hazard.

8. Air Brakes
Foundation Brake Components:
Disc Brakes
 Disc brake systems are becoming popular
 Less maintenance, and much lighter weight = saving$

9. Air Brakes
Foundation Brake Components:
Disc Brakes
 Air in the service chamber pushes the internal lever to move the
“bridge” (which acts much like a caliper piston in hydraulic brakes) to
exert force on the inboard brake pad.
 Since the caliper “floats” on two pins, the movement of the caliper
forces the outer brake pad against the rotor -- this clamping action of
the brake pads on the rotor applies braking force to the wheel

10. Air Brakes
Components of Air Brakes:
Air System
 The diagram below illustrates the routing of airlines/valves
on a truck

11. Air Brakes
Components of Air Brakes:
Air System
 The diagram below
illustrates the
routing of airlines
and components
on a truck/trailer

12. Air Brakes
Components of Air Brakes:
Air Supply Subsystem
 The air supply system consists of the following:
 Air compressor, belt driven or mounted directly to the
engine (gear driven)
 Governor
 “Wet” air tank
 Pop-off safety valve
 One-way check valve
 Air dryer (optional) Air governor
mounted on
air compressor

13. Air Brakes
Components of Air Brakes:
Air Supply Subsystem – Air Compressor
 As the piston moves downward,
air is drawn into the cylinder

14. Air Brakes
Components of Air Brakes:
Air Supply Subsystem – Air Compressor
 As the piston moves upward,
air is “pushed” out of the
cylinder

15. Air Brakes
Components of Air Brakes:
Air Supply Subsystem – Air Compressor
 Once maximum pressure is
reached (120 psi), the
governor directs air pressure
to the unloader port of the
air compressor
 Basically the “Unloader”
functions by keeping the
intake valve open, thus
no longer creating compressed
air.

16. Air Brakes
Components of Air Brakes:
Air Supply Subsystem – Air Governor
 The air governor is adjusted to direct air to the unloader port of
the air compressor, usually around 120 to 125 psi
 The cut-in pressure is around 20 to 25psi below the cut out value

17. Air Brakes
Components of Air Brakes:
Air Supply Subsystem – Air Tanks (reservoir
tanks)
 In accordance with Document #121, three air tanks are required.
 Wet or Supply tank.
 Primary tank .
 Secondary tank

18. Air Brakes
Components of Air Brakes:
Air Supply Subsystem – Air Tanks (reservoir
tanks)
 Must store 12 times combine volume of all service brake chambers
at maximum travel of diaphragms.
 All tanks under Document #121 requires drain taps. (drain daily)
 Tested at 500psi or 3448kpa.(Safety)
 Two compartments in one tank. ( Wet and Primary)
 Tanks are located usually under or around the frame of the vehicle.

19. Air Brakes
Components of Air Brakes:
Air Supply Subsystem – Air Dryer (Optional)
 Many air brake systems utilize an air dryer
 Air dryers also act as a “filter” to
maintain a clean and dry air system.
 “Automatically” purges collected moisture
by the air directed to the purge valve from
the unloader port of the air governor

20. Air Brakes
Components of Air Brakes:
Air Supply Subsystem – Air Dryer (Optional)

21. Air Brakes
Components of Air Brakes:
Air Supply Subsystem – Safety Pop Off Valve
 The safety pop off valve is used on
the airbrake system to allow excess
air pressure to release once
pressure reaches 150psi.
 This valve is located any where in
the supply circuit. (supply tank)
 If leaking check the unloading
circuit of the supply circuit.
 Malfunctioning valves are rare.

22. Air Brakes
Components of Air Brakes:
Primary and Secondary Subsystem
 The dual circuit design means that in case of air loss in one
circuit the brakes will still be apply on those wheels connected to
the other circuit.
 The signaling pressure in both the primary and the secondary
circuits are metered from the foot valve (treadle valve).
 Metered air: Metered air means signal pressure ( applied ) from
the foot valve. (Treadle)
 The relay valves will get the same metered signal from the foot
valve. (relay valve)

23. Air Brakes
Components of Air Brakes:
Primary and Secondary Subsystem
 System fills the service air tanks first, ensuring that enough air is
available to operate the service brakes before spring brakes are
released through a check valve. (trailers only).

24. Air Brakes
Components of Air Brakes:
Primary and Secondary Subsystem
 Since most brake-system designs use dual circuits, air is drawn
from both the primary and secondary tanks and is directed to
specific wheels on the vehicle.
 The wheels that receive air through the primary or secondary
circuits vary, depending on the vehicle manufacturer.
 The dual-circuit design means that if one circuit fails, the brakes
will still operate on the wheels connected to the other circuit.

25. Air Brakes
Components of Air Brakes:
Primary and Secondary Subsystem – Operation
 Pressing the brake pedal allows the driver to
control the air pressure applied to the
service brakes ( METERED AIR).
 Brake feel in the pedal comes from a rubber
block and spring.
 Air enters and exits to the two circuits.
(Primary and Secondary)
 The primary piston pushes the relay
(secondary) piston with a cushion of air.

26. Air Brakes
Components of Air Brakes:
Primary and Secondary Subsystem – Operation
 Relay valves are primarily used on long wheelbase vehicles to
apply and release rear axles service or parking brakes.
 They are air operated, graduating control valves of high capacity
and fast response.
 Upon signal pressure from the service brake valve, they
graduate, hold or release air pressure from the chambers to
which they are connected.
 They are generally mounted close to the chambers they serve.

27. Air Brakes
Components of Air Brakes:
Primary and Secondary Subsystem – Operation
 Relay valves are available in both remote and reservoir mount
designs and feature inlet/exhaust valve cartridge replacement
without line removal.

28. Air Brakes
Components of Air Brakes:
Primary and Secondary Subsystem – Operation
 Metered air (controlled from brake pedal/treadle valve) is
directed to the service chambers. The service chambers exert
force through a push-rod to move the slack adjuster (S-Cam) or a
lever (disc)

29. Air Brakes
Components of Air Brakes:
Primary and Secondary Subsystem – Operation
 Brake chamber size: Sized by the effective sectional area of the
diaphragm.
 Measured in square inches. ( 30/30 series)

30. Air Brakes
Components of Air Brakes:
Primary and Secondary Subsystem – Operation
 When air enters the brake chamber through the inlet port it acts
upon the diaphragm, pushing the plate and pushrod assembly
forward. The pushrod then acts upon the slack adjuster which
moves the S-cam assembly .
 The yoke is connected to a slack adjuster, which in turn is
connected to the brake cam shaft.
 The greater the air pressure admitted to the brake chamber the
greater the force applied by the pushrod.
 Pressure is determined by multiplying the delivered air pressure
by the effective diaphragm area.
 Example:
 If 60psi is admitted to the chamber that has a diaphragm of 30
square inches the lineal force on the end of the pushrod is
approximately 1800 pounds.

31. Air Brakes
Components of Air Brakes:
Primary and Secondary Subsystem – Operation
 Adjusters are used to connect the brake chamber with the S- cam
shaft and allow for adjustment of brakes.
 Slack adjusters are the mechanical link between the brake
chamber and the foundation brake assembly.
 The adjuster is attached to the brake chamber push rod by
means of a clevis and pin.
 The amount of leverage potential depends on the
distance between the clevis pin and the fulcrum
formed at the centerline of the S- cam.
 This is known as “ LEVERAGE FACTOR”.

32. Air Brakes
Components of Air Brakes:
Primary and Secondary Subsystem – Operation
Out of adjustment

33. Air Brakes
Components of Air Brakes:
Spring Brake Subsystem
 Must provide a fail safe or emergency braking function to all air
brake equipped vehicles stated by document #121.
 Combining emergency braking with the vehicle service brakes.
(dual brake chambers)
 Spring brake chambers apply the brakes by means of a large coil
spring.
 To release the spring brakes 60psi of air pressure must be
supplied to the spring brake chambers.

34. Air Brakes
Components of Air Brakes:
Spring Brake Subsystem
 DO NOT TAMPER WITH CHAMBER. Only remove “service”
portion of brake chamber – CAGE spring brake first before
removing.
 Exerts force up to 1800psi (spring force)
 https://www.youtube.com/watch?v=wgRPTOK8Q9k

35. Air Brakes
Helpful websites:
 Ontario Ministry of Transportation:
The Official Air Brake Handbook, helpful basics created to help
drivers to complete the “Z” endorsement of a commercial vehicle
drivers’ license.
http://www.mto.gov.on.ca/english/handbook/airbrake/index.shtml
 Youtube:
Be selective for a reputable source, Youtube has some very good
videos that are informative.

36. Air Brakes
Finished?
 Complete the 15 question quiz to complete this training module.
 This training module is only for awareness, any work completed
on an air brake system should done under the guidance of an
experienced/qualified technician. (Personal injury and vehicle
damage may occur otherwise.)
The End