CDL · Air Brakes · 6 min read
Understanding Separate Air Circuits
- Identify the components, layout, and axle assignments of primary and secondary air brake circuits.
- Explain the pressure requirements for pre-trip buildup and the operating thresholds for low air pressure warning devices.
- Describe the vehicle handling effects of pressure loss in one circuit and the mandatory driver actions required if a low pressure warning activates.
The Purpose and Architecture of Dual Air Brake Systems
Operating a heavy commercial vehicle requires an exceptionally reliable method for slowing and stopping. For this reason, most heavy-duty vehicles use dual air brake systems for safety. A dual air brake system has two separate air brake systems that use a single set of brake controls. These two distinct systems work together during daily operations to ensure that the commercial motor vehicle maintains dependable stopping performance under demanding road conditions.
Even though the system is split into two independent circuits, the driver does not have to manage multiple pedals or levers while driving down the highway. Both systems in a dual air brake system use a single set of brake controls. The two separate air brake systems use a single set of brake controls. The driver uses one set of brake controls for the two separate air brake systems.
The separation between these two circuits is thorough and physical. Each system in a dual air brake setup has its own air tanks, each system has its own hoses, and each system has its own air lines. Each system has its own air tanks, hoses, and lines; if one system is very low on pressure, either the front or rear brakes will not operate fully. Dual air brake systems share a single set of brake controls despite having separate circuits, giving commercial drivers an important safety buffer.
Common mistakes
- Thinking that dual air brake systems have two separate brake pedals, when they actually share a single set of brake controls.
- Believing it is acceptable to drive when only one of the two air systems has reached 100 psi, instead of waiting for both systems to build at least 100 psi.
- Assuming the low air pressure warning activates at 100 psi rather than before pressure drops below 55 psi in either system.
- Continuing to drive to a repair shop after the low air warning light and buzzer activate, instead of stopping right away and safely parking the vehicle.
Key points
- Most heavy-duty vehicles use dual air brake systems for safety, featuring two separate systems known as the primary and secondary systems.
- Dual systems share a single set of brake controls, but each system has its own air tanks, hoses, and air lines.
- The primary system typically operates regular brakes on the rear axle or axles, while the secondary system operates regular brakes on the front axle and possibly one rear axle; both supply air to a trailer.
- Drivers must allow the air compressor to build up a minimum of 100 psi in both the primary and secondary systems before driving, monitoring either two gauges or two needles in one gauge.
- The low air pressure warning light and buzzer should come on before pressure falls below 55 psi in either system and should shut off when pressure in both systems rises to a manufacturer-set value that must be greater than 55 psi.
- If one system has very low pressure, front or rear brakes will not operate fully, increasing stopping distance and making it take longer to stop.
- If the low air pressure warning activates while driving, the driver must stop right away, safely park the vehicle, and have the system fixed.
Practice this section
Explained CDL Air Brakes questions, no account needed. CDL Air Brakes practice test
The full lesson continues with 4 more sections with exam tips, audio in English and Spanish, 5 Check yourself questions and 18 flashcards.
- 1 month
$44.99$22.49 - 3 months
$131.99$65.99 - Lifetime
$269.99$134.99
One payment, no subscription.
See CDL plansAlready a member? Log in
Sample lesson: Making and Storing Compressed Air25 practice questions on this exam
Each CDL practice test has 25 original questions with the answers explained. No account needed.
Open the CDL practice testsOriginal lesson written for CDL practice · updated October 31, 2026. Exam names are trademarks of their owners; tests.global is not affiliated with them.