The supply pressure gauge shows how much pressure

• A. Is in the air tanks
• B. Is in the air lines
• C. Is in the hydraulic lines
• D. Is in the fuel system

Answer: (A)

The supply pressure gauge shows how much air pressure is stored in the air tanks that power the braking system. This stored air provides the force needed to apply and release the brakes, so keeping enough pressure in the tanks is essential for safe operation. It doesn’t measure pressure in the air lines that run to the brake chambers, nor does it relate to hydraulic lines or the fuel system. In many systems you’ll see other gauges for line pressure, but the supply gauge is specifically about the tanks’ stored air.