1. Air Brake Systems

2. Air Brake System Color Key
Introduction
The purpose of an air brake system on heavy duty vehicles is to convert air pressure to mechanical energy to activate the foundation brakes. Federal Motor Vehicle Safety Standard (FMVSS) 121 dictates how this is to be done for over-the-road vehicles.
The purpose of this presentation is to help you construct Meritor WABCO Truck and Tractor air systems. It will also help you identify air system components and understand how they relate to FMVSS 121.
NOTE: The systems described in this workbook are generic air brake systems.
Included in this presentation are:
Truck Systems
Air Supply
Secondary Service
Primary Service
Parking
Anti-Compounding
Emergency (Prior to 3/1/98)
Emergency (After 3/1/98)
COLOR KEY
SUPPLY
SECONDARY
PRIMARY
PARK / EMERGENCY
TRAILER CONTROL
TRAILER SUPPLY
Air Brake System Color Key
Tractor / Trailer Systems
Tractor Parking & Trailer Air Supply
Trailer Control
For additional information, call ArvinMeritor's Customer Service Center at

3. Truck Air Supply System
Compressor
The compressor supplies all of the air for the brake system and any accessory requiring air. Compressors rely on the vehicle's engine to be driven, cooled, lubricated and sometimes for an intake air source. It works on the same principle as a piston engine. Meritor WABCO has two models: a 318 single cylinder and a 636 dual cylinder. Both have reed type valving and a sliding leaf valve unloader. FMVSS 121 requires that a compressor be of sufficient capacity to increase air pressure in the supply and service reservoirs from 85 p.s.i. to 100 p.s.i. when operating at maximum R.P.M. in approximately 25 seconds.
To prevent compressor problems always maintain an unrestricted area at the compressor inlet and discharge.
* Meritor WABCO has both single and twin cylinder compressors to meet your air requirements. *

4. Truck Air Supply System
Air Dryer
Air dryers are not required, but are common on most air braked vehicles today. The Meritor WABCO air dryer has a spin-on desiccant. The dryer's main job is to remove moisture and contaminants which are created while compressing air. Two basic styles available: a system purge style and an external purge style. Both systems use single cartridge designs and are available in a 1200 and 1800 model. The system purge style uses air from the supply (wet) and secondary tanks to purge (back flush) the dryer. The external purge style uses air from its own external mounted tank which is isolated from the brake system.
All desiccant style dryers require regular maintenance (desiccant cartridge).
Meritor WABCO also has a twin dryer available for high air volume use vehicles, greater than 25 cfm.

5. Truck Air Supply System
Supply Tank
The supply tank (wet tank) is needed to collect any moisture or contaminants not removed by the dryer and to make up a portion of the total storage volume required by FMVSS 121. The supply tank can also be used to indicate when the dryer requires service.
Note: A supply tank is not required if the primary and secondary tanks both have automatic or manual drain valves.

6. Truck Air Supply System
Pressure Relief Valve
The pressure relief valve, also known as a pop-off or safety valve, is needed to protect the system from excessive pressure.
Most valves are pre-set to open at 150 p.s.i.

7. Truck Air Supply System
Automatic / Manual Drain Valve
Drain valves (spitter valves) are required on primary and secondary reservoirs if the vehicle is not equipped with a supply (wet) tank. The automatic drain valve works on an internal pressure differential which means the valve momentarily expels (spits) moisture every p.s.i.. Most drain valves are equipped with a manual drain and are available in either heated (12 volt) or non-heated versions.

8. Truck Air Supply System
Low Pressure Switch
Low pressure switches are required by FMVSS 121 to give the driver a visual indication, other than a gauge, anytime system pressure is below 60 p.s.i. when the ignition is in the on position. By regulation it must be visual but can be audio and visual.

9. Truck Air Supply System
Governor
The governor is needed to control the compressor's out-put and purge the dryer. The governor is basically an on / off switch, but is adjustable like a regulator.
A governor's typical operating pressure range is 25 p.s.i. i.e., 130 / 105 p.s.i. (cut-out / cut-in).
FMVSS 121 requires that for buses built after March 1, 1997 the cut-in pressure must be 85 p.s.i. or greater, and for truck / tractors 100 p.s.i. or greater.
When one end of the operating range is adjusted, the other end changes approximately the same amount.

10. Truck Air Supply System
Discharge
UNL
RES
Should not be more than1/4” ID
Inlet
UNL
Inlet
Delivery

11. Truck Secondary Service System
Pressure Control Check Valve (PCCV)
The pressure control check valve is used on the secondary reservoir in conjunction with a system purge style air dryer. The PCCV allows air to flow freely into the system reservoir and back to the supply (wet) tank above 95 p.s.i. The PCCV is typically installed on the service reservoir in place of the one-way check valve. The PCCV maintains minimum air pressure in the service reservoir and allows the air dryer to use 10 p.s.i. of air volume from the supply (wet) tank and secondary tank during regeneration.
FMVSS 121 requires that each service reservoir system be protected against loss of air pressure due to failure or leakage in the system between the service reservoirs and the source of air pressure by a check valve or equivalent device.

12. Truck Secondary Service System
Secondary Service Tank
The secondary service tank is used to provide an isolated storage area for air to be used for service and emergency braking.
The combined volume of all service and supply reservoirs must be at least 12 times the combined volume of all service chambers at maximum stroke.

13. Truck Secondary Service System
Manual Drain Valve
The manual drain valve is required by FMVSS 121 to be on each reservoir of an air brake system so the driver can drain the system daily.

14. Truck Secondary Service System
Air Gauge
The air gauge is required by FMVSS 121 so the driver can monitor the pressure for each service system when seated in the normal driving position. The manufacturer can install two separate gauges, one for primary and one for secondary, or they can install a duplex (dual needle) gauge.

15. Truck Secondary Service System
Dual Foot Control Valve
The dual foot control valve provides the driver modulated control of the vehicle's service brakes. There are three different styles of dual foot control valves. One thing common to all is that the primary section is normally the section closest to the pedal and is mechanically applied. The secondary is the furthest section away from the pedal and is controlled from primary air. If primary air is not present (failed primary system), the secondary is mechanically applied.
Generally, the primary section applies 3-5 p.s.i. of air, prior to the secondary section starting to apply.
Most (but not all) manufacturers send secondary air to the steer axle, primary air to the drives and dominant air to the trailer.

16. Truck Secondary Service System
Quick Release Valve
The quick release valve releases air pressure from the air chambers quickly to assist in meeting FMVSS 121 release timing requirement. Some quick release valves are used to aid in balancing application pressure. Ensure you are using the correct one when replacing quick release valves.
Quick release valves may have a differential pressure. Differential pressure is the difference between the inlet pressure compared to the outlet pressure.
Example: This quick release valve has an approximate 3 p.s.i. differential. If you put 10 p.s.i. to the inlet, it delivers approximately 7 p.s.i. etc.
The letters and numbers QRV-030 identify this valve. They can be found stamped in the body next to the part number or on the tag.

17. Truck Secondary Service System
ABS Modulator Valve
The ABS modulator valve modulates the air pressure in the brake chamber if wheel lock-up is imminent. During normal braking air passes directly through the valve. If the ABS is actuated the valve will modulate chamber pressure.
Additional ABS modulator valve information is available in Meritor WABCO maintenance manuals.

18. Truck Secondary Service System
Service Chambers
A service chamber is used during braking and converts air pressure to linear motion or mechanical force.
Service chambers are available in standard and long stroke models. Normally, the port outside diameter of a standard stroke chamber is round. In the long stroke stroke chamber the port outside is square plus the chamber is identified as long stroke.
The components are not interchangeable.

19. Truck Secondary Service System
Manual Slack Adjusters
Manual slack adjusters convert linear motion from the chamber to rotary motion of the s-cam.
Manual slack adjusters are used to maintain proper brake adjustment when replacing a chamber.
Manual slack adjusters are known as either bi-directional or uni-directional.
A bi-directional slack adjuster can be installed either way on an s-cam. With the adjusting nut facing toward or away from the chamber.
A uni-directional slack adjuster must be installed with the adjuster nut facing away from the chamber or damage could occur.

20. Truck Secondary Service System
Automatic Slack Adjusters
As it's name implies, the automatic slack adjuster does everything the manual slack does, only automatically if properly installed.
The automatic slack adjuster saves downtime. It should be checked periodically to ensure a problem is not present.
Automatic slack adjusters were mandated on all new vehicles manufactured on or after October 20, 1994.

21. Truck Secondary Service System
#2 Delivery port
#2 Supply Port

22. Truck Primary Service System
One-Way Check Valve
If the vehicle is equipped with a system purge style dryer, a PCC valve is normally used on the secondary service reservoir and a one-way check valve is used on the primary reservoir.
A one-way check valve allows air to flow from the supply source into the primary reservoir, but does not allow air to flow in the opposite direction.
FMVSS 121 requires the service reservoirs to be protected against the loss of air pressure.
If a vehicle is not equipped with a system purge style dryer, both the primary and secondary reservoirs will have one-way check valves.

23. Truck Primary Service System
Primary Service Tank
Same as secondary service system and is also used in the primary service system.
Manual Drain Valve
Air Gauge
Dual Foot Control Valve
ABS Modulator Valve
Manual Slack Adjusters
Automatic Slack Adjusters
Same as secondary service system and are also used in the primary service system.

24. Truck Primary Service System
Service Relay Valve
A service relay is utilized to send a modulated signal from the foot valve to the service chambers when sending air longer distances or for better air balance.
A relay valve works much like a quick release valve except it has a reservoir line which is not on a quick release. The reservoir or supply line is connected directly to a reservoir.
Whereas quick release valves may have differential pressure relay valves may have crack pressure. Crack pressure is similar to differential pressure except that differential pressure is throughout the whole range of the valve. Crack pressure is only present in a relay during the initial stages of the application. It is the amount of air it takes to open the valve before it delivers any air.

25. Truck Primary Service System
Spring Brakes
Spring brakes enable service braking capabilities and parking braking capabilities from a single unit.
The push rod comes out of the service side end of the chamber.
The other end of the spring brake is known as the spring or emergency side. Do not mess with this end. There is a spring with approximately 1800 lbs. of force inside this chamber. A spring brake is the most dangerous thing on a heavy duty truck.
Spring brakes are also available in standard and long stroke and are identified the same as service chambers.

26. Truck Primary Service System
Two-Way Check Valve
Two-way check valves are used in air brake systems to monitor two pressures and deliver the highest.
Two-way check valves are available in two different styles: Shuttle-type which is mounting sensitive the inputs must be horizontal.
Diaphragm type which is not mounting sensitive - can be installed any way. 1. 2.

27. Truck Primary Service System
Stoplight Switch
The stoplight switch is mandated by FMVSS 121 and requires that the brake lights activate before chamber pressure reaches 6 p.s.i.
This switch is normally open.

28. Truck Primary Service System
#1 Supply Port
#1 Delivery
Port

29. Truck Parking System Two-Way Check Valve
FMVSS 121 states that in the event either system (primary or secondary) fails you still must have the capabilities of releasing the vehicle parking brakes. The two-way check valve meets this requirement by having both primary and secondary system pressure at the two-way check and allowing dominant air to flow to the dash control valve.

30. Truck Parking System Dash Valve (Yellow Diamond Knob)
The dash control valve or commonly called the push-pull valve is needed to control the release and application of the vehicle parking brakes.
By regulation this valve must be identified as to what it controls and how it controls it.
The standard form of operation is to push to release and pull to apply. Releasing delivers air pressure to the parking chamber, releasing spring force. Applying exhausts air pressure from the parking chamber, applying spring force.

31. Truck Parking System Quick Release Valve with Double Check
FMVSS 121 states that from the instant the driver pulls the dash valve, the parking brakes should be 100% mechanically applied in less than 3 seconds.
Since the quick release valve always has a zero differential it aids in meeting FMVSS 121 application timing of the spring brakes

32. Truck Parking System Spring Brakes How a spring brake functions:
Step #1:
With no air on the spring side or the service side, the vehicle is parked by mechanical force.
Step #2
With air pressure on the spring side only, the parking brakes are released and the vehicle is ready to roll.
Step #3
With air pressure on both the spring side and the service side, the vehicle is stopped with service air pressure.

33. Truck Parking System
Two Way Check Valve

34. Truck Anti-Compounding System
Quick Release with Two-Way Check Valve
The quick release with two-way check valve is used for anti-compounding but not mandated by law.
In an air brake system anti-compounding prevents the service and the parking brakes from being applied at the same time.
The following is an example of how much force could be produced by a spring brake.
The spring in the spring chamber produces approximately 1800 lbs. of force. If the vehicle has 30" / 30" spring brakes without anti-compounding, is parked and the push rod is connected to a 6" slack arm:
1800 x 6 = 10,800 in lbs. torque
At the same time, the driver steps on the brake pedal and delivers 50 p.s.i. to the service side of the spring brake:
50 p.s.i. x 30" chamber = 1500 lbs. force x 6" slack = 9,000 in. lbs. additional = this is compounding

35. Truck Anti-Compounding System (Cont'd.)
Quick Release with Two-Way Check Valve (Cont'd.)
With the preceding example, having anti-compounding as the driver applied pressure on the service side, the same pressure would be delivered to the spring side, releasing that amount of spring force.
Things that could possibly break if the vehicle does not have anti-compounding:
(1) Premature cracking or breaking of brake drums
(2) Bend chamber push rods
(3) Rip the mounting studs out of the spring brakes
(4) Break slack adjusters
(5) Break spring brake mounting brackets off the axle

36. Truck Anti-Compounding System

37. Truck Emergency System
Inversion Valve (Prior to March 1, 1998)
Meritor WABCO Production Part Number
Meritor WABCO Service Part Number RKN28030
Required on all straight trucks and buses manufactured between 1975 and March 1, Under normal conditions, this valve does not have to work. The only time this valve must function is when there has been a primary system failure (reservoir to zero). At that time, the parking brakes are released with secondary air and the vehicle has service braking with secondary air on the steer axle. The inversion valve receives the modulated secondary signal from the foot valve and equally releases air pressure from the spring chamber giving the drive axle a mechanical brake application. As the driver disengages the brakes, a secondary signal will re-release that mechanical application.

38. Truck Emergency System
New Inversion Valve (Used After March 1, 1998)
Meritor WABCO Production Part Number
Meritor WABCO Service Part Number RKN28032
The new inversion valve is required on all straight trucks and buses and functions exactly the same as the previous valve except it will also function with a control line failure in the primary system. Previous valve may be used if a two control line primary relay valve is used.
To check either valve for function on a vehicle:
With the vehicle dash valve knob pushed in and wheels blocked, build up the air system completely. Shut vehicle off.
Drain the supply (wet) tank - see if the one-way checks are working.
Drain the primary tank.
Two people are needed, one in the driver's seat, the other back by the drive axle.
The driver gradually depresses the foot valve, the technician in the back should hear air exhausting and see the chamber push rods extending.
As the driver disengages the brake pedal, the push rods should retract.
(1)
(2)
(3)
(4)
(5)
(6)

39. Truck Emergency System Meritor WABCO Production Part No. 9718990010
Add an R-12 DC?

40. Truck Emergency System Meritor WABCO Production Part No. 9718990020

41. Tractor Parking and Trailer Air Supply System
Module Dash Control (Yellow Diamond Knob & Red Octagon Knob)
The two-way check for the parking system is built internally. The yellow diamond knob controls the application of the total vehicle parking brakes and the release of module dash control of the tractor parking brakes. The red octagon knob controls the release and application of the trailer parking brakes plus the fill of the trailer service reservoirs.
Quick Release With Two-Way Check Valve
Like a straight truck system, this valve prevents two forces from working on the foundation brakes at the same time.

42. Tractor Parking and Trailer Air Supply System
(2 Line) Tractor Protection Valve
Tractor protection valves in general are required by FMVSS 121 to protect the tractor's air system from the effects of loss of air on the trailer or loss of trailer. Two types are available:
(3 Line) Tractor Protection Valve
Could be automatic or non-automatic style. Non-automatic style (non-flow sensitive): It will not sense any pressure drop. It relies on the dash valve popping when the line pressure reaches about p.s.i.
The difference between the two line and three line tractor protection valves other than one additional air line, is that the three line protection valve will release the trailer's parking brakes and fill the trailer's reservoirs 35% quicker than a two line.
Could be automatic or non-automatic style. Automatic style (flow sensitive); If it senses a pressure drop, it shuts off the air automatically somewhere between 75 and 100 p.s.i.

43. Tractor Parking and Trailer Air Supply System
(Trailer Supply) Gladhand
Gladhands are the quick couplings between the tractor and trailer in which air passes through for the trailer.
Universal and polarized gladhands are manufactured. Universal will connect to each other. Polarized are color coded and / or identified service and emergency.
If I had a polarized (color coded) supply gladhand, what color would it be?
Answer - Red Supply is emergency (Red) - control is service (Blue)

44. Tractor Parking and Trailer Supply System

45. Tractor Trailer Control System
Hand Control Valve
Hand control valves are not required by any regulation in the U.S. or Canada.
This valve has several slang names (Trolly Valve, Spike Valve, Johnson Bar and Broker Valve). It allows the driver to operate the trailer service brakes without using any tractor brakes.
There are many major fleets today who spec their tractors without hand control valves because better stopping is accomplished with all brakes doing their share of the work.
(2) Two-Way Check Valves
If either service system (primary or secondary) on the tractor fails, the combination vehicle will still have service braking from the good system on the tractor as well as the trailer system.

46. Tractor Trailer Control System
Stoplight Switch
The stoplight switch is required by FMVSS 121 to activate the brake lights prior to service chamber pressure reaching 6 p.s.i.

47. Tractor Trailer Control System

48. Tractor Air System Complete