If the brakes do not release after you push the trailer air supply valve, what should you check?

When the trailer air supply valve is pressed and the brakes do not release, it indicates a potential issue with the air brake system, particularly in how air is being supplied to the trailer's brake mechanism. Checking the air line connection becomes crucial because if there is a disconnection, leak, or blockage in the air lines, air cannot travel from the tractor to the trailer, preventing the brakes from functioning correctly.

If the air line isn't connected securely, or if there is damage to the line itself, the system cannot build air pressure needed to release the brakes. Proper air flow is essential for the overall operation of the braking system, making this check vital for ensuring safety and functionality.

The other options, like engine oil level, tire pressure, and trailer load distribution, are important vehicle safety checks in their own right but do not directly relate to the immediate issue of brakes not releasing in the context of an air brake system.