An air brake system is actually three separate braking systems working together. Which one applies and releases the brakes during ordinary driving, each time the driver presses and releases the brake pedal?

Explanation

Pressing and letting go of the brake pedal during ordinary driving is handled by the service brake system, the one that applies and then releases the brakes on every stop. An air brake system is actually three systems sharing components: the service brake system for everyday stops, the parking brake system that holds the vehicle in place once parked, and the emergency brake system, which borrows parts from the other two to bring the vehicle to a halt if the air supply itself fails. 49 CFR 383.111(b)(1) requires CDL applicants to know general air brake system nomenclature, the required-knowledge area this question falls under; the specific description of what each system does is standard driver-training content from the manual, not spelled out word for word in the CFR paragraph itself. The parking brake system is wrong because it engages only when the driver sets its control, not throughout normal driving. The emergency brake system is wrong because it is reserved for an actual failure of the air supply, not routine stops. Antilock brakes are wrong because ABS is an add-on to the normal braking system that helps keep the wheels from locking up; it isn't one of the three systems that actually applies or releases the brakes.

Source: 49 CFR 383.111(b)(1)