CDL Air Brakes Comprehensive Study Guide

Air Brake System Combination and Core Components

• Modern heavy vehicle air brake systems combine three distinct systems into one integrated setup:

  • Service Brake System: Applies and releases the brakes when the driver depresses the brake pedal during normal driving.

  • Parking Brake System: Applies and releases the parking brakes when the vehicle is parked.

  • Emergency Brake System: Utilizes parts of the service and parking brake systems to bring the vehicle to a stop if the primary service brake system fails.

  • Subsystem Distinction: The spring brake system is an underlying mechanism for parking and emergency braking, but it is not classified as an independent air brake subsystem.

• Foundation Brakes:

  • S-Cam Drum Brake: The most common type of foundation brake found on heavy commercial vehicles.

  • S-Cam Operation: The S-cam rotates to force the brake shoes outward against the inside of the brake drum.

  • Friction Surface: When brakes are applied, the brake shoes press directly against the inner surface of the brake drum or disc.

  • Inspection and Safety Thresholds: It is unsafe and illegal to drive a vehicle equipped with brake drums that have cracks longer than $\frac{1}{2}$ of the width of the friction area.

• Driver Control Interface:

  • Brake Pedal: Directly controls the amount of air pressure applied to put on the brakes.

  • Pressure Depletion Risk: Repeatedly and partially pressing and releasing the brake pedal (fanning the pedal) can result in a critical loss of air pressure.

  • Stop Light Switch: An automatic switch that turns on the vehicle's rear brake lights whenever the air brakes are applied.

Air Compressor, Governor, and Pressure Metrics

• Air Compressor Governor:

  • Controls when the air compressor pumps air into the air storage tanks.

  • Cut-In Pressure: The pressure level at which the governor allows the compressor to start pumping air, normally set to $100\,\text{psi}$.

  • Cut-Out Pressure: The pressure level at which the governor stops the compressor from pumping air, normally set between $125\,\text{psi}$ and $150\,\text{psi}$ (with standard full charge achieved at $125\,\text{psi}$).

• System Charging and Testing Standards:

  • Dual Air Brake System Build-Up Rate: In a properly functioning dual air brake system with minimum size tanks, running the engine at a fast idle should build air pressure from $85\,\text{psi}$ to $100\,\text{psi}$ within $45\,\text{seconds}$.

  • Insufficient Build-Up Warning: If air pressure does not build up within the correct time frame, pressure may drop to a dangerously low level while driving, requiring an immediate emergency stop.

Pressure Gauges, Safety Valves, and System Protections

• Dash Pressure Gauges:

  • Supply Pressure Gauge: Required on all air brake-equipped vehicles; continuously displays the amount of air pressure remaining in the air storage tanks.

  • Application Pressure Gauge: Shows the exact amount of air pressure being applied to the brakes when the brake pedal is depressed.

• System Safety Mechanisms:

  • Safety Relief Valve: Set to discharge automatically if pressure in the system reaches $150\,\text{psi}$ to protect tanks and lines from over-pressurization.

  • Safety Valve Malfunction Sign: If the safety relief valve opens or discharges several times, the air brake system requires immediate repair and attention.

  • One-Way Check Valve: Installed between the compressor and storage tanks to prevent air from escaping the tanks if the air compressor develops a leak.

Air Storage Tanks, Moisture Management, and Cold Weather Care

• Moisture and Oil Contamination:

  • Compressed air naturally contains water and compressor oil, which collect at the bottom of air storage tanks.

  • Risk of Un-drained Water: Water can freeze in cold weather, causing valve malfunction and complete brake failure.

  • Drain Frequency: If the vehicle is not equipped with automatic air tank drains, the driver must manually drain the tanks at the end of each day of driving.

• Alcohol Evaporators:

  • Function: Introduced into the air system to inject alcohol vapour, which lowers the freezing point of moisture and reduces the risk of ice forming in air brake valves during cold weather.

  • Daily Maintenance: During cold weather, drivers must check and fill the alcohol level daily.

  • Consequence of Neglect: Failing to maintain proper alcohol levels can lead to ice formation in storage tanks and valves, causing total brake failure.

Spring Brakes and Parking Emergency Controls

• Spring Brake Mechanics:

  • Normal Operation: Mechanical springs are held retracted by compressed air pressure while driving.

  • Automatic Activation: If air pressure drops to a low threshold—specifically below $20\,\text{psi}$ in straight truck chambers—the spring brakes automatically engage to bring the vehicle to a stop.

  • Holding Force: Parking and emergency brakes are legally held in position by mechanical spring pressure because springs cannot leak away like air pressure.

  • Braking Power Dependency: The stopping power of spring brakes depends directly on the service brakes being kept in proper mechanical adjustment.

  • Pedal Compounding Hazard: Never push the brake pedal down when the spring brakes are applied, as compounding forces can damage brake components.

• Parking Brake Controls and Protocols:

  • Engagement Method: Under normal conditions, engaging the parking brake releases the air pressure holding back the springs.

  • Dashboard Knob: Modern vehicles use a yellow, diamond-shaped push-pull knob on the dash to control parking brakes.

  • Usage Mandate: Parking brakes must be used any time the vehicle is parked or left unattended. Leaving a vehicle unattended without applying parking brakes and chocking wheels is never permissible.

  • Exception / Caveat: Do not apply parking brakes when the brakes are extremely hot (e.g., after descending a steep grade), as excessive heat can crack or warp the brake drums. Allow hot brakes to cool first.

• Special Auxiliary Valves:

  • Dual Parking Control Valves: Vehicles equipped with dual parking control valves can draw pressure from a separate auxiliary tank to temporarily release spring emergency/parking brakes to move the vehicle a short distance in an emergency.

  • Front Brake Limiting Valves: Found on older commercial vehicles manufactured prior to 1975.

Low Air Pressure Warnings and System Leakage Testing

• Low Air Pressure Warning Devices:

  • Activation Threshold: A low air pressure warning device must activate before pressure in the service air tanks falls below $60\,\text{psi}$.

  • Warning Types: Visual signals (lights/buzzers) or mechanical devices such as a wig wag (a drop-down mechanical arm).

  • Driver Action: If a low air pressure warning activates for one or both systems, stop the vehicle right away, park safely, and do not continue driving until the issue is repaired.

• Air Leakage Rate Limits (Engine Off):

  • Straight Truck / Bus (Brakes Released): Air pressure loss must not exceed $2\,\text{psi}$ in $1\,\text{minute}$.

  • Straight Truck / Bus (Brakes Applied): Air pressure loss must not exceed $3\,\text{psi}$ in $1\,\text{minute}$.

Inspection, Operational Adjustments, and Driving Techniques

• Service Brake Pre-Trip Test:

  • Park on level ground, build air pressure to normal operating levels, release the parking brake, roll forward slowly at approximately $5\,\text{mph}$, and apply the brake pedal firmly. The vehicle should stop smoothly without pulling to either side.

• Checking Manual Slack Adjusters:

  • Park on level ground, chock the wheels, and release the parking brakes to remove tension from the components.

  • Pull hard on each slack adjuster; if it moves more than approximately $1\,\text{inch}$ where the push rod attaches, it requires mechanical adjustment.

• Mountain Driving and Downgrade Techniques:

  • Primary retarding power on long, steep downgrades comes from the braking effect of the engine (low gear selection). Service brakes serve only as a supplement.

  • Proper Braking Technique: Once the vehicle is in the proper low gear, allow speed to build to the safe speed, apply service brakes firmly until speed drops $5\,\text{mph}$ below the safe speed, then release the brakes completely. Repeat as necessary (e.g., if safe speed is $40\,\text{mph}$, brake down to $35\,\text{mph}$ before releasing).

• Brake Heat, Expansion, and Fade:

  • Excessive use of service brakes generates extreme heat, causing brake drums to expand away from the shoes.

  • Brake Fade: Defined as the situation where a driver must push progressively harder on the brake pedal to maintain speed on a downgrade, potentially resulting in total brake failure.

• Brake Lag Distance and Emergency Stopping:

  • Brake Lag: Air brakes take longer to react than hydraulic brakes because air takes more time to flow through lines and build pressure in chambers compared to hydraulic fluid.

  • Distance Added: At $55\,\text{mph}$ on dry pavement, air brake lag adds approximately $32\,\text{feet}$ to stopping distance.

  • Total Stopping Distance: At $55\,\text{mph}$ under ideal conditions, total stopping distance for an air brake-equipped truck or bus is more than $300\,\text{feet}$.

  • Emergency Stop Maneuver: Brake in a manner that preserves steering control and keeps the vehicle moving in a straight line.

  • Emergency Stab Braking: Press the brake pedal as hard as possible; when the wheels lock up, release the brakes completely; once the wheels resume rolling, reapply the brakes fully.