Air Brake System Study Notes

Importance: Air disc brakes should only be adjusted or repaired by a qualified mechanic.

Supply Line: The air line that supplies air to the trailer at full tractor reservoir pressure is referred to as the supply line.

Manual Slack Adjuster Travel: During a brake application, the travel should not exceed 1¾ inches.
Brake Fade: The main cause of brake fade at high temperatures is the expansion of the brake drums away from the linings.

Alternative Units: Besides PSI (Pounds per Square Inch), air can also be measured in KPa (Kilopascals).

Function: The spring brake modulator valve provides controlled rear braking in the event that the primary system fails.

Dusted Border Signs: Signs with dusted borders indicate content specifically pertaining to commercial drivers.

Holding Power Test: To test the holding power of spring brakes, a driver should complete a tug test in low gear with the spring brakes applied. This acts as a safety check for the holding strength of the tractor and trailer's parking brakes by gently pulling against them.
Testing Location: Brake adjustment on an air brake-equipped vehicle can be checked from underneath the truck and trailer.

Main Components: The primary components of an air brake system include:
- Reservoir
- Governor
- Compressor
- Air Lines
- Brake Valve (Foot Valve)
- Foundation Brakes

Force Calculation: If 100 PSI is applied to an area of 30 square inches, the force produced is:
- Formula: $ext{Force} = ext{Pressure} imes ext{Area}$
- Calculation: $100 ext{ PSI} imes 30 ext{ in}^2 = 3000 ext{ lbs}$

Maximum Operating Pressure: Most air brake systems operate at a maximum pressure of 125 PSI.
Governor Settings:
- The governor allows the compressor to resume compressing air when it drops below 80 PSI.
Compressor Build Time: The compressor must build pressure between 50-90 PSI within 3 minutes.
- Maximum Pressure Range: Different vehicle types may have maximum pressure parameters that vary between 105-135 PSI.

Low Pressure Restart Point: If maximum air pressure is 125 PSI, the governor will restart the compressor if air pressure drops to 105 PSI.
Minimum and Maximum Pressure Range: The range between minimum and maximum pressure is 20 PSI.
Emergency Restart Pressure: The governor must restart the compressor if air pressure drops below 80-85 PSI.

Pop-Off Valve: This is also referred to as the safety valve.
Foot Valve: The brake pedal is sometimes referred to as the foot valve treadle.

One-Way Check Valve: This valve prevents the total loss of air in the compressor discharge line in the event of a break.
Pre-Trip Inspection: Drivers are required by law to do a pre-trip inspection for safety reasons.
Governor Reload Pressure: The compressor must reload at no lower than 80 PSI.

Best Method: The best method for descending a long and steep downgrade is to use the correct gear and engine retarder, and refrain from relying solely on the brakes.

Supply Line: The emergency line is also referred to as the supply line.
California Spring Parking Brake: Spring parking brake emergency release systems may be referred to as the California spring parking brake system.

Safety Valves: The safety valve is located on top of the air reservoir, while the drain cock is located at the bottom.
Reservoir Drying: Allow plenty of time for the reservoirs to completely dry before use.
Wet Tank: The reservoir referred to as the “wet tank” is the supply reservoir.

Primary Tank: Air from the primary tank is applied to the rear axle brakes.
Secondary Tank: Air from the secondary tank is applied to the front axle brakes.

Activation: The low air warning device gets activated when air pressure drops below 60 PSI.
Types: Different types of low air warning devices include:
- Warning indicator light on the dashboard
- Buzzer
Visual Warning: Some older vehicles feature a low air warning device near the top of the windshield that drops into the driver’s view when air pressure drops below approximately 60 PSI.

Relay Valve Location: The relay valve is located between the reservoir and rear brake chambers.
Purpose: Relay valves are used to reduce lag time when applying brakes.

Spring Brake Mounting: Spring brakes are typically mounted on the rear axles.
Reaction Time: Defined as the time it takes for a driver to apply the brake after a hazard has been recognized.

Brake Chambers in Air-Over-Hydraulic System: In an air-over-hydraulic system, brake chambers act as pressure intensifiers.
Bobtailing: Driving a tractor without a trailer is referred to as bobtailing.
Manual Slack Adjuster Procedure: When adjusting a manual slack adjuster, the adjusting nut should be turned to tighten the brake by rotating the S-cam in the same direction as the brake application.
Spring Parking Brake System Function: The spring parking brake emergency release system allows a driver to move a disabled vehicle to safety.
Use of Front Wheel Limiting Valve: This is used as necessary on slippery roads to reduce front wheel brake application by 50%.
Control Line Alternative Name: Another name for the control line is service line.
Compounding Braking Applications: The best way to avoid compounding braking is to release the spring brakes before making a heavy service brake application.
Following Distance: In a bus, truck, or heavy vehicle, the following distance should be no less than 5 seconds.
Final Movement Determinant: The final factor that will determine if a vehicle will move forward is traction.
Air Brake Endorsement: A driver must have an air brake endorsement to operate a vehicle equipped with air-over-hydraulic brakes unless the system does not use an air track.