During the final pre-trip air brake check, how does a driver confirm the spring brakes will engage automatically at the correct pressure?

Explanation

A driver confirms the spring brakes will engage automatically by releasing the parking brake and, on a combination vehicle, the tractor protection valve, then continuing to bleed air pressure down by cycling the brake pedal; both valves ought to pop out or snap shut, setting the spring brakes, once pressure falls into the 20 to 45 psi range. 49 CFR 383.111(b)(5)(i) specifically requires CDL applicants to know about automatic fail-safe devices as part of safe and accurate pre-trip inspection procedures, a direct match since spring brakes are exactly that kind of device; the 20-45 psi range itself is reused from 49 CFR 393.43, which sets that activation band for a trailer's automatic brake feature. Manually pulling the parking brake knob and timing it is wrong because that tests the manual control, not whether the spring brakes engage automatically from falling air pressure. Testing only while moving is wrong and unsafe; this check is performed with the vehicle stationary and chocked. Disconnecting the compressor is wrong because the test relies on gradually cycling the brake pedal to bleed pressure down in a controlled way, not cutting off the air supply entirely.

Source: 49 CFR 383.111(b)(5)(i)