Alabama CDL Class A Permit Practice Test 2026 - Free CDL Permit Practice Questions and Study Guide

The thing being tested is how we know how much air pressure is available in the system. The supply pressure gauge is the instrument that shows the pressure in the air tanks and lines, so you can tell at a glance whether there is enough pressure to operate the brakes and other air-powered components. This gauge is essential for safety because if the pressure is too low, braking performance can be reduced and the system may not function properly.

The other parts aren’t used to measure pressure: the air filter cleans the incoming air, the brake chamber uses the air pressure to push the brakes, and the slack adjuster sets the clearance of the brakes. So the supply pressure gauge is the component that communicates the system’s air pressure level.