Final answer:
In pre 1975 single air systems, air pressure should build from 50-90 PSI within 3 minutes, which is crucial for the efficiency and safety of vehicle air brake systems.
Step-by-step explanation:
In single air systems (pre 1975), air pressure should build from 50-90 PSI within 3 minutes. This is a standard for air brake systems in vehicles, which need to be capable of building up sufficient pressure within a short period to ensure the safety and functionality of the brakes. In this context, PSI refers to pounds per square inch, a unit of pressure.