To successfully pass your CDL test, you will need to have the air brakes check completed. This is not an optional requirement, it’s a safety protocol that all professional drivers should be knowledgeable about. During your CDL test, you will be asked to show and explain every step of the air brake check on your tractor-trailer.
We understand how stressful it can be to pass this important test. That’s why we’ve put together this detailed guide to help you confidently perform an air brakes check.
Continue reading to discover the correct way to set up your vehicle and how to test the air pressure systems. You’ll learn the steps involved in conducting leak rate tests and ways to verify warning systems. We’ll also cover techniques for checking the functionality of the parking brake.
Whether you’re getting ready for your CDL test or simply brushing up on your knowledge, this comprehensive guide will teach you how to carry out your air brakes check like a pro.
Setting Up for the Air Brakes Check
Before you start your air brakes check, proper vehicle setup is crucial for both safety and test success. Here’s what you need to do:
1. Safe Start Procedure
- Put your vehicle in neutral
- Push in the clutch
- Verify parking brakes are set
- Start the vehicle
2. Air Pressure Requirements
- Check both primary and secondary air pressure systems
- Ensure readings are above 100 PSI
- Normal operating pressure range: 100-125 PSI
- Allow pressure to build if readings are low
Pro Tip: If your secondary system shows low pressure (around 40 PSI) while the primary system reads above 100 PSI, wait for both systems to reach proper operating pressure before proceeding.
Air Pressure Build-Up Process
You may accelerate the pressure build-up by increasing engine RPM to approximately 1,200. When increasing this, observe carefully both pressure gauges until they reach 125 PSI. At this point, listen carefully for the characteristic sound of air dryer, which indicates governor cut-out. Also, observe low air warning alarm, which should switch off when pressure is between 60 and 65 PSI.
These initial setup procedures form the foundation for a successful air brake test and help discourage potential test problems or safety risks.
Conducting the Initial Air Pressure Test
Your air brake system in your tractor-trailer consists of two essential components – the primary and secondary systems. There needs to be appropriate pressure levels in both systems to ensure safe operation.
Normal Operating Pressure Range:
- Minimum: 100 PSI
- Maximum: 125 PSI
To perform your initial air pressure check:
- Start with both systems at full pressure
- Watch your dashboard gauges:
- Primary system gauge
- Secondary system gauge
If either system drops below 100 PSI, let the air pressure build back up. You’ll notice the low air warning alarm automatically shuts off once pressure reaches 60-65 PSI – this confirms your warning system is functioning correctly.
Quick Pressure Build-up Tip: Increase engine RPM to about 1,200 to help build pressure faster. Keep an eye on both gauges until they reach 125 PSI.
At maximum pressure (125 PSI), listen for the air dryer to release excess pressure – it makes a distinctive spitting sound. This indicates your governor has cut out and the air compressor has stopped pumping air, exactly what you want to see.
Remember: Both primary and secondary systems must maintain equal pressure above 100 PSI before proceeding with any further brake system tests. Uneven pressure between systems could indicate potential issues that need immediate attention.
Conducting the Leak Rate Test
The leak rate test is a crucial component of your CDL test requirements. Here’s how to perform it correctly:
1. Set up your vehicle:
- Push in the clutch
- Place the vehicle in first gear
- Push in both brake knobs (yellow parking brake and trailer air supply)
- Turn off the engine
- Release the clutch
- Turn key to “on” position without starting the vehicle
2. Begin the test:
- Press and hold the brake pedal firmly
- Let the air pressure stabilize
- Watch both primary and secondary gauges for any pressure drops
- Listen for potential air leaks
Your gauges should maintain steady pressure throughout the test. Any noticeable drops indicate potential system leaks that need immediate attention.
CDL Test Requirements
During your CDL test, you must bring your own timing device to accurately measure the duration of various procedures. You’ll need to hold the brake pedal for precisely one minute while conducting the air brake test. Both pressure gauges must remain stable throughout this period, showing no significant drops in pressure. Additionally, there should be no audible air leaks present during the test, as this would indicate a system malfunction that needs addressing before the vehicle can be safely operated.
Pro Tip: During your CDL test, verbally announce when you start timing and when the minute is complete. This demonstrates your attention to detail and understanding of the procedure.
The leak rate test ensures your air brake system maintains proper pressure under load – a critical safety feature that prevents brake failure during operation.
Checking the Low Air Warning Alarm
After completing your leak rate test, you’ll need to verify your low air warning system’s functionality. Here’s how to perform this critical safety check:
1. Keep your setup ready:
- Brake knobs pushed in (yellow and red)
- Ignition key in the “on” position
- Engine can be off during this test
2. Begin the pressure drop:
- Pump the brake pedal repeatedly
- Watch both primary and secondary pressure gauges
- Monitor for warning indicators
3. Watch for warning signals at 60 PSI:
- Red warning light on the gauge
- Audible alarm (when engine running)
- Either primary or secondary system may trigger first
Safety Note
During actual driving conditions, a low air warning is a serious alert. If you see these warning signals on the road, you must immediately locate a safe area to pull over and stop your vehicle while brake control remains effective. It’s crucial to avoid continuing to drive with low air pressure. Once stopped, wait for pressure to build back up before proceeding with your journey.
Your low-air warning system serves as your first line of defense against brake failure. A properly functioning system should activate at or above 60 PSI, giving you enough time to respond safely.
Testing Parking Brake Functionality
Your truck’s parking brake system is an essential safety feature that serves as an emergency brake when air pressure drops too low. Here’s how to properly test these critical components:
1. Continue Pumping the Brake Pedal
- Keep the truck’s key in the “on” position
- Maintain steady pumping to reduce air pressure
- Watch both the gauges and brake knobs simultaneously
2. Monitor Brake Knob Activation
- The trailer brake knob (red) should pop between 20-45 PSI
- Your tractor brake knob (yellow) activates in the same range
- Both knobs must pop out independently
3. Safety Check Requirements
- Record the PSI readings when each knob activates
- Listen for any unusual sounds during activation
- Check for smooth, decisive knob movement
- Ensure knobs remain locked in the out position
A properly functioning emergency brake system will automatically engage if your air pressure drops dangerously low during operation. This fail-safe design prevents brake failure and protects you on the road.
Choose Mac’s Diesel and Trailer Repair
Mastering the air brakes check on your tractor trailer requires practice, attention to detail, and proper knowledge. While this guide can take you through it, safety has to be your first priority at all times. If you have doubts about making these inspections or require professional assistance, our professionals at Mac’s Diesel and Trailer Repair are at your service.
Our Specialties
We offer comprehensive professional truck repair to ensure your fleet keeps on running effectively. Our professional technicians offer comprehensive DOT inspections to maintain your fleet in all safety regulations and standards. In case of unforeseen breakdowns, rely on our prompt emergency roadside assistance to have you back on the road in no time. Our experienced bus and RV repair is available for performing routine maintenance and challenging mechanical problems for any recreational and commercial vehicle.
Need expert help with your tractor trailer? Our mobile diesel mechanics come directly to you in Lexington, Kentucky, and surrounding areas. Give us a call at (859) 433-4062 to schedule an appointment or discuss your truck repair needs.