What is the typical fully charged pressure for an air brake system?

• A. 125 psi
• B. 110 psi
• C. 100 psi
• D. 135 psi

Answer: (A)

In an air brake system, the tanks are charged by the compressor until the governor stops the pressure rise at the cut-out level. For most heavy vehicles, that cut-out and thus the fully charged pressure is about 125 psi. This provides a solid reserve of stored air for braking and auxiliary components, and it’s the standard pressure the system is designed to reach and hold.

Choosing 125 psi makes sense because it represents the maximum pressure the system normally operates at; lower numbers like 110 psi or 100 psi would indicate a partially charged system with less braking reserve and sooner loss of pressure during operation. A value like 135 psi is beyond typical design for standard air brakes.