If one of the two systems in a dual air brake system loses most of its pressure while the vehicle is being driven, what happens?

Explanation

If one of the two systems in a dual air brake system loses most of its pressure, whichever axle that system supplies, front or rear, stops braking at full strength, and the vehicle needs a longer distance to stop. 49 CFR 383.111(b)(4) requires CDL applicants to know the implications of low air pressure readings, and this is a direct example of that knowledge area: a dual system is built so a failure in one circuit does not take down the other, but it does reduce the vehicle's total braking capacity. Automatic air transfer between systems is wrong because the two systems are deliberately kept separate; there is no mechanism that moves air from one to the other. Saying only the parking brake is affected is wrong because the failing system's service brakes, on whichever axle it controls, are what actually lose full function. Total loss of braking on every axle overstates the failure; the entire point of a dual system is that the other, unaffected system keeps working.

Source: 49 CFR 383.111(b)(4)