Dual air brake systems, explained
Section 5.2 of the Mississippi CDL Manual (p. 5-5) explains dual air brake systems, which most heavy vehicles use. It is tested on the Air Brakes test.
Key facts
| Part of | the Air Brakes test (80% to pass) | 49 CFR 383.135 |
|---|---|---|
| Manual section | §5.2 Dual air brake systems | MS CDL Manual p. 5-5 |
| Questions in CDL Mile's bank | 12 | CDL Mile, checked against MS CDL Manual |
Key rules
- § 5.2p. 5-5
In a typical dual system the primary works the rear axle brakes, and the secondary works the front axle (and possibly one rear axle). If the secondary is very low, the front brakes won't be working fully.
Dual air brake systems
- § 5.2p. 5-5
Give the compressor time to build at least 100 psi in both the primary and the secondary systems before you drive.
Dual air brake systems
- § 5.2p. 5-5
You need at least 100 psi in both the primary and the secondary systems before driving. The secondary is only at 85 psi, so wait until it builds to 100 psi or more.
Dual air brake systems
- § 5.2p. 5-5
As the compressor builds air, the warning light and buzzer should go off once pressure in both systems reaches a value set by the manufacturer. That value must be greater than 55 psi.
Dual air brake systems
Try two questions
Air Brakes · Dual air brake systems
Why will it take you longer to stop if one half of a dual air brake system is very low on pressure?
Tap an answer to see the explanation and the manual page.
Air Brakes · Dual air brake systems
You are driving a truck with dual air brakes. The primary gauge looks normal, but the warning light and buzzer come on because the secondary is low. What should you do?
Tap an answer to see the explanation and the manual page.
More Air Brakes topics
Sources
Mississippi Commercial Driver's License Manual, version March 2025, Mississippi Department of Public Safety. Page references are to its printed page numbers. CDL Mile is independent and not affiliated with Mississippi DPS.
Updated for the March 2025 Mississippi CDL manual. Last checked November 5, 2026.
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