How is a tractor brake test performed?

The procedure for a tractor brake test involves releasing the red valve, which controls the trailer brakes, while keeping the yellow valve, which regulates the tractor brakes, engaged. This method allows the driver to test the functionality of the tractor's air brakes without affecting the trailer's braking system. By maintaining engagement of the yellow valve, you can effectively test the tractor hydraulic actuation and assess if the brakes are working correctly when pressure is applied.

In contrast, other methods like keeping both valves completely engaged or releasing them entirely do not accurately simulate the operating conditions necessary for a thorough brake functionality test. Engaging both valves while pushing on the brakes might not isolate the tractor's braking system from the trailer's, making it challenging to assess their individual performances. Thus, correctly performing the tractor brake test involves the specific action of releasing the red valve while keeping the yellow valve engaged to isolate and properly evaluate the tractor brakes.