Modern air brakes are pretty simple if you realize that there are really two separate systems involved. The emergency (or park) system's job is to keep the springs in the brake pods compressed while the truck is in motion. If your truck does not have dual chamber (spring) brakes you only need to concern yourself with the service brake circuit (with a few changes). The treadle valve (brake pedal) sends air to a relay valve. This relay valve opens and allows air stored in an air tank to flow to the service brakes which applies them. When the brakes are released the relay valve closes, shutting off the flow of air to the brake pods from the tank WHILE dumping the air pressure in the lines from the relay valve to the brake pods. It sounds to me like a relay valve is missing or stuck open.

On the older trucks, when the brake valve is released, air pressure from the tanks goes to the brakes to hold the vehicle from moving but when the air leaks off, the brakes release and the truck rolls away. That is why you have a manual parking brake on this truck. When hooking up to an older trailer without spring brakes, you had to first connect the air lines and charge the brake system before attemping to hook up or else the trailer would roll away from you unless the wheels were securely chocked.

Hope this helps. I think if you study your current brake schematics and isolate the service brake components, you will be able to figure out what items are needed.

Remember 9-11-01--God Bless the USA
JUSTICE, not REVENGE, will prevail

1951 Chevy 1/2-ton Pickup truck