In a well-maintained air brake system, at what pressure should the air compressor start pumping?

In a well-maintained air brake system, at what pressure should the air compressor start pumping?

• A. 60 psi
• B. 100 psi
• C. 80 psi
• D. 120 psi

Answer: (B)

Air brake systems use a governor to cycle the compressor between cut-in and cut-out pressures. The compressor starts pumping when system pressure falls to the cut-in point, which is typically about 100 psi, and stops at the cut-out point (around 125 psi). So 100 psi is the correct starting pressure because it’s the threshold at which the governor signals the compressor to begin building air again, ensuring there’s enough pressure for braking and other functions while avoiding excessive cycling. Pressures like 60 or 80 psi are too low for safe braking, and 120 psi would delay pumping until pressure already dropped further, narrowing the usable range.

Air brake systems use a governor to cycle the compressor between cut-in and cut-out pressures. The compressor starts pumping when system pressure falls to the cut-in point, which is typically about 100 psi, and stops at the cut-out point (around 125 psi). So 100 psi is the correct starting pressure because it’s the threshold at which the governor signals the compressor to begin building air again, ensuring there’s enough pressure for braking and other functions while avoiding excessive cycling. Pressures like 60 or 80 psi are too low for safe braking, and 120 psi would delay pumping until pressure already dropped further, narrowing the usable range.