‘77 4070B, dog leg and brake release valves. Hooked the truck to my hopper bottom trailer and it won’t release just the trailer brakes. Both truck and trailer have to be released. I can’t imagine them not working independently. When I try to apply the dog leg, alot of air is released from the valve under the brake pedal. I swapped that valve with another, from my ‘78 4070B parts truck and it did the same thing. Checked another 4070, and the valve release air from the bottom also. Really got me scratching my head.