What is a dual air brake system?

A two brake system, one operates the front brakes and one operates the back brakes

Well it depends on what the previous answer is referring to; But when it comes to large class A commercial vehicles the following apply:

A. A dual air brake system has two separate air brake systems which use a single set of brake controls

B. A dual air brake system has two separate air brake systems, the first system is called the primary system. The other is called the secondary system.

C. A dual air brake system has two separate air brake systems, each system has its own air tanks, hoses, lines, etc..

D. A dual air brake system has two separate air brake systems, both systems supply air to the trailer if there is one.

If you see this question on a CDL written test and the answers are like these and then they have an E. All of the above, the answer is E.