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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. 

## Subsection 5.1 Test Your Knowledge 

## 5.2 – Dual Air Brake 

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. 

