Final answer:
First, check the airline connections for proper attachment and leaks. If the airlines are fine, then inspect the electrical cable, and do not cross the airlines as it is unsafe.
Step-by-step explanation:
If the brakes are not released when you push the trailer air supply valve, the first step is to check the airline connections. Ensuring that the airlines are properly connected and there are no leaks is essential for the proper functioning of the brake system on a trailer. If the airlines are in good condition and correctly connected, the next step is to inspect the electrical cable, which can affect the functionality of the trailer brakes if not properly connected. Crossing the airlines is never a recommended action as it can lead to dangerous situations by providing incorrect signals to the trailer's braking system.