CDL Air Brakes Dual Air Brake Systems Summary

Most heavy vehicles use dual air brake systems for safety. A dual system has two separate air brake systems, which use a single set of brake controls. Each system has its own air tanks, hoses, lines, and so on.

Wrong choices

• One air system with two compressors: A dual system has two separate air systems, not just two compressors.
• An air brake system with both spring brakes and service brakes: All air brake vehicles have both; that is not what "dual" means.

Before driving a vehicle with a dual air system, allow time for the air compressor to build up a minimum of 100 psi pressure in both the primary and secondary systems. Watch the primary and secondary air pressure gauges (or needles, if the system has two needles in one gauge).

Wrong choices

• At least 60 psi in either system: 60 psi is the low air warning level; it is not enough to start driving.
• At least 100 psi in the primary system only: Both systems must reach at least 100 psi.

The warning light and buzzer should come on before air pressure drops below 60 psi in either system. If this happens while driving, you should stop right away and safely park the vehicle. Have the air system repaired before driving on.

Wrong choices

• Keep driving, because the other system still has full pressure: With one system low, part of your brakes will not work fully; you must stop.
• Pump the brake pedal until the warning shuts off: Pumping the pedal uses air and makes pressure drop faster.

In a dual air brake system, one system, called the "primary" system, usually operates the regular brakes on the rear axle or axles. The other, called the "secondary" system, operates the brakes on the front axle (and possibly one rear axle). Both systems supply air to the trailer (if there is one).

Wrong choices

• The front axle brakes only: The front axle brakes are usually on the secondary system.
• Only the trailer brakes: Both systems supply air to the trailer; the primary system's main job is the rear axle brakes.

The "secondary" system usually operates the brakes on the front axle (and possibly one rear axle). The primary system usually works the rear axle brakes. Both systems supply air to the trailer.

Wrong choices

• All the rear axle brakes: The rear axle brakes are usually on the primary system.
• Only the parking brakes: The secondary system operates service brakes, mainly on the front axle.

A dual air brake system has two separate systems. Each system has its own air tanks, hoses, lines, and so on, but both use one set of brake controls (one brake pedal). This way, a leak in one system does not drain the other.

Wrong choices

• Its own brake pedal: Both systems share a single set of brake controls.
• Its own steering wheel: Steering is not part of the air brake system.

In a dual air brake system, there is a gauge for each half of the system, or a single gauge with two needles. Watch both readings: one for the primary system and one for the secondary system.

Wrong choices

• With one needle that shows the average of both systems: Each system's pressure must be shown on its own, not averaged.
• Only by listening for the governor to cut out: The governor sound does not tell you the pressure in each system.

Pay attention to the low air pressure warning light and buzzer. The warning light and buzzer should shut off when air pressure in both systems rises to a value set by the manufacturer. This value must be greater than 60 psi.

Wrong choices

• As soon as either system rises above 60 psi: Both systems must be above the set level before the warning shuts off.
• Only when both systems reach the governor cut-out pressure: The warning shuts off at a lower level set by the manufacturer, not at cut-out.

Each system works a different set of brakes. If one air system is very low on pressure, either the front or the rear brakes will not be operating fully. That is why you must stop and park safely as soon as the low air warning comes on.

Wrong choices

• All the brakes keep working at full power because the other system takes over: The good system cannot fully replace the lost one; one set of brakes is weakened.
• The spring brakes come on right away on every axle: Spring brakes depend on pressure dropping to about 20 to 45 psi; losing one system weakens the brakes first.

Before driving, both the primary and secondary systems must reach at least 100 psi.

1. Primary: 110 psi, which is at least 100 psi.
2. Secondary: 80 psi, which is below 100 psi.
3. So keep waiting until the secondary needle also reaches 100 psi.

Check: if the secondary pressure does not rise, have the system checked before you drive.

Wrong choices

• Start driving, because the primary system is above 100 psi: Both systems must reach at least 100 psi, not just one.
• Start driving, because both systems are above 60 psi: 60 psi is the warning level, not the minimum to start driving.