1. Truck Brake Systems

2. Highway Tractor Air Brake System with ABS

3. Trailer Air Brake System with ABS

4. Air Brake Sub-systems The air supply circuit Primary circuit
Secondary circuit
Dash control valves–parking/emergency circuit
Trailer circuit
Foundation brakes

5. Pre-ABS Tractor, Trailer, and Dolly

6. Supply Circuit Components

7. Typical Air Compressors

8. Shop Talk
FMVSS No. 121 requires that governor cut- out be set within the range of 115 psi to 135 psi. The ATA TMC recommends that governor cut-out be set at 125 psi.
It is a legal requirement that governor cut-in occur at no more than 25 psi less than governor cut-out pressure.
If the difference between governor cut- out and cut-in is less than 20 psi, the result is too frequent cycling of the compressor loaded and unloaded cycles.

9. Typical Air Dryer Installation

10. Shop Talk Air dryers must be fitted with a safety valve.
Because the system safety valve is located in the supply tank, if the hose from the dryer to the supply tank got plugged (with ice or contaminants) or kinked, the governor would not cut out the compressor effective cycle, and the resulting high pressure could explode the air dryer.

11. Warning
Alcohol evaporators MUST be located downstream from the air dryer in the supply circuit.
Alcohol will turn air dryer desiccant into mush if pumped through the system.

12. An Older Truck’s Service and Parking Brake Circuits

13. Tractor Emergency Circuit Pre-ABS System

14. Pre-ABS Spring Brake Valve Circuit

15. Dash Control Valves

16. Shop Talk
Make sure you understand the function of each dash control valve in the system.
Block the wheels and get into a truck with a fully charged air system.
Use the controls and observe the effect.

17. Tandem Axle Trailer Service and Parking/emergency Circuits

18. Foundation Brake Types
S-CAM
WEDGE
Disc

19. Cam Geometry
“S” Cam
“Flat” Cam

20. Brake Shoe Rollers

21. Wedge-actuated Brake System

22. Air Disc Brake

23. Dual Circuit Application/Treadle Valves

24. Typical Dual Circuit Brake Valve

25. Shop Talk
One OEM survey of warranty returns indicates that over half of the foot valves submitted function perfectly.
This misdiagnosis results from a failure by the technician to understand how the valve operates normally.
Take some time to understand exactly how this important valve masters the operation of the service brakes.

26. Relay Valves

27. Primary Circuit

28. Quick Release Valve

29. Ratio Valves

30. Caution
If a tractor is converted to a straight truck, the bobtail proportioning valve must be replaced with a non- proportioning relay valve.
In a case in which a long-wheelbase tractor was converted to a wrecker, the operator complained of lazy service braking until the treadle was floored, when the brakes would lock. Replacement of the bobtail proportioning valve with a standard relay valve rectified the problem.

31. Caution
Because air is delivered to the trailer hold-off circuit before the trailer tanks are charged, the trailer can be moved before any service braking is possible.
You should charge trailer tanks before attempting to move the trailer.

32. Spring Brake Circuit

33. Shop Talk
Current spring brake control valves must prioritize air to the hold-off circuit over air to the air tanks.
This means that when air builds up in the air system, it is first delivered to the hold-off circuit, enabling the release of the parking brakes.
This enables a vehicle with a disabled air supply system to be moved much more quickly. A much smaller volume of air has to be transferred to the vehicle before release of the parking brakes becomes possible.

34. Tractor Protection Circuit

35. Shop Talk
FMVSS No. 121 requires that the Tractor protection valve isolate the tractor air supply from that of the trailer when pressure drops to between 20 and 45 psi.

36. In-line Single Check Valve

37. Double Check Valve

38. Air Brake Chambers

39. Cutaway of a Typical Air Chamber

40. Typical Spring Brake Chamber

41. Caution The spring force in a spring brake chamber can kill.
Never take any risks with spring brake chambers.

42. Spring Brake Chamber Operation

43. Manual and Automatic Slack Adjusters

44. Stroke-sensing Automatic Slack Adjuster

45. Shop Talk
Slack adjusters without a grease fitting should not be assumed to be functional.
Slack adjusters should be lubricated with a low-pressure grease gun.

46. Note
Around 95 percent of braking on a loaded tractor/trailer combination involves brake application pressures of 25 psi or less; 85 percent of braking involves brake application pressures of 15 psi or less.
axle and hub removal
Inspection
brake install