Questions and Answers
What is a dual air brake system?
A dual air brake system has two separate air brake systems, which use a single set of brake controls.
What are slack adjusters?
Slack adjusters are parts for an S-Cam brake system that cause the brake shoes to push outward when the brake is pressed.
How can you check slack adjusters?
Park on level ground and chock the wheels, then release the parking brakes and pull hard on each slack adjuster.
How can you test the low pressure warning signal?
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How can you check that the spring brakes come on automatically?
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What are the maximum leakage rates?
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How do you perform the air compressor governor cut-out pressure test?
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How do you perform the air compressor governor cut-in pressure test?
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Study Notes
Dual Air Brake System
- Consists of two separate air brake systems controlled by a single set of brake controls.
- Each system has distinct components: air tanks, hoses, and lines.
- Typically, one system operates the rear axle's brakes, while the other operates the front axle's brakes.
- Both systems provide air supply to the trailer.
Slack Adjusters
- A component in the S-Cam brake system that adjusts brake shoe position.
- When brakes are applied, the push rod extends the slack adjuster, causing the cam shaft to rotate, pushing brake shoes against the drum.
- Slack adjusters that extend over one inch when pulled require immediate adjustment to prevent brake failure.
Checking Slack Adjusters
- Ensure the vehicle is on level ground and wheel chocked.
- Release parking brakes and pull on accessible slack adjusters.
- Any movement over one inch at the push rod attachment indicates the need for adjustment.
Testing Low Pressure Warning Signal
- Shut off the engine after attaining sufficient air pressure to avoid warning signal activation.
- Turn on electrical power and depress brake pedal to lower air tank pressure.
- The warning light should illuminate when pressure drops between 55-75 psi.
Verifying Automatic Spring Brake Activation
- Chock the wheels and release all parking brakes.
- Reduce air pressure by repeatedly pressing the brake pedal.
- The tractor protection valve and parking brake valve should activate when air pressure drops to manufacturer-specified levels (20-45 psi), engaging the spring brakes.
Maximum Leakage Rates
- For single vehicles: maximum leakage should not exceed 3 psi in one minute.
- For combination vehicles: maximum leakage should not exceed 4 psi in one minute.
Air Compressor Governor Cut-out Pressure Test
- Run the engine at fast idle, monitoring rising air pressure in the system.
- The governor should cut out before the pressure reaches 130 psi.
- The point at which the pressure stabilizes indicates the governor cut-out pressure.
Air Compressor Governor Cut-in Pressure Test
- With the engine idling, pump the brake pedal to decrease air tank pressure.
- Observe the air pressure gauge to identify the pressure at which the compressor reactivates.
- The cut-in pressure must be no lower than 85 psi for buses and 100 psi for trucks.
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Test your knowledge on dual air brake systems and air brake inspections with these flashcards. Each card provides key definitions and concepts essential for understanding the workings and regulations of air brake systems in vehicles. Perfect for students and professionals preparing for exams.
