Spring brakes serve as the emergency and parking brake in an air brake system. Within what air pressure range do they begin to activate automatically as air pressure drops?

Explanation

Spring brakes activate automatically once air pressure drops into a range between 20 and 45 psi, with most vehicles set to apply them around 20 to 30 psi, using the mechanical force of a heavy spring rather than air pressure to push the brakes on. This range comes directly from 49 CFR 393.43, which requires every trailer's brakes to apply automatically on breakaway and requires a tractor towing an air-braked trailer to have an automatic activation feature that triggers within a fixed pressure band no lower than 20 psi and no higher than 45 psi, matching the WA manual's figure closely. 60 to 80 psi is wrong because that range is well above where spring brakes engage, closer to normal operating pressure. Activating only at exactly 0 psi is wrong because spring brakes are designed to apply well before air pressure is fully exhausted, precisely so the vehicle does not lose braking entirely. Requiring a manual driver action is wrong because the entire safety purpose of spring brakes is that they apply automatically, without the driver doing anything, if the air supply fails.

Source: 49 CFR 393.43