During brake system failure, the emergency brake relies on a combination of both systems. It uses the same wheel brake components (the parts involved in the service brakes) and the parking brake mechanism that provides a spring-applied, air-released means to apply those brakes when the normal air pressure is lost. This setup ensures you still have braking capability even if the primary service brake system isn’t functioning. The ABS system isn’t part of this emergency stopping method, so it isn’t the means by which the emergency brake stops the vehicle.

Measuring the air in the tanks is what this item tests. The device that shows how much air is stored in the tanks is the supply pressure gauge. It displays the current pressure in the air reservoir system, usually in psi, so you can confirm there’s enough pressure to safely operate the brakes and other air-powered systems. The brake pedal is used to apply braking force, but it doesn’t measure pressure. The tractor protection valve is a safety device that isolates air to the trailer if the supply drops; it doesn’t provide a pressure reading. The wheel cylinder is part of the braking mechanism that uses air pressure to push the brake shoes, not a measuring instrument.

Spring brakes are spring-applied, air-released. The air pressure in the brake chambers counteracts the force of the release springs, keeping them retracted and the brakes released during normal operation. If air pressure drops, the springs push out to apply the brakes. Hydraulic pressure, oil temperature, or tire pressure don’t control this release mechanism.

In a dual air brake system, there are two independent air circuits that control braking. If the low-air warning comes on for one circuit, that circuit doesn’t meet the safe pressure required for reliable braking. Relying on the other circuit isn’t enough, because a failure in the first circuit could lead to reduced or total braking capability at any moment. So the safest action is to stop as soon as you can safely, park, and only continue after the issue is repaired and pressures are restored to the proper levels. Continuing to drive toward a stop or driving slowly while ignoring the warning risks a sudden loss of braking on that circuit. Ignoring the warning altogether is unsafe because a single system with insufficient pressure can compromise stopping power, especially in emergencies.