What does the term 'cut-in' refer to in air brake systems?

The term 'cut-in' in air brake systems refers specifically to the pressure point at which the system begins to utilize air for the braking process. In this context, the correct answer indicates that at 90 psi, the air pressure is sufficient for the brake system to activate and start engaging. This is critical for drivers to understand, as it provides a baseline for when the braking system is operational and can effectively begin to apply pressure to the brakes.

Knowledge of the 'cut-in' pressure helps ensure that drivers are aware of their system's performance and readiness. Understanding this point is crucial for ensuring safety, as operating below this pressure may lead to inadequate braking response. In contrast, other terms in the options, such as brake failure rate and reduction of braking force, are unrelated to the specific activation point of the system and do not provide insight into the effective operation of air brakes. The reference to 120 psi concerns the 'cut-out' point, not the 'cut-in', thus reinforcing the focus on the initial activation at 90 psi.