He said that you release the trailers brakes. It doesn't make alot of sense if there's no air in the towed truck, the brakes won't work. He should of put the air system directly to the entire truck.
Because the service line doesn't work if the parking brake isn't released. He's feeling the blue line from the tow vehicle to the front chambers of the vehicle being towed thats why the towed vehicle doesn't need air in it.
