2005 Model Commercial Driver’s License Manual Figure 5.4 5.2 – Dual Air Brake  Subsection 5.1  Test Your Knowledge  Most heavy-duty vehicles use dual air brake  systems for safety. A dual air brake system has  two separate air brake systems, which use a single  set of brake controls. Each system has its own air tanks, hoses, lines, etc. One system typically  operates the regular brakes on the rear axle or  axles. The other system operates the regular brakes on the front axle (and possibly one rear  axle). Both systems supply air to the trailer (if there  is one). The first system is called the "primary"  system. The other is called the "secondary"  system. See Figure 5.4.  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). 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.  Page 5-5  Section 5 – Air Brakes  1.  Why must air tanks be drained?  2.  What is a supply pressure gauge used for?  3. All vehicles with air brakes must have a low air  pressure warning signal. True or False?  4.  What are spring brakes?  5. Front wheel brakes are good under all conditions. True or False?  6. How do you know if your vehicle is equipped  with antilock brakes?  These questions may be on your test. If you can’t  answer them all, re-read subsection 5.1. 