Trailer air supply.

I have an international dump truck that I am trying to get the trailer brake supply working on. I cannot get the supply valve to stay activated. The red knob will not stay pished in. I have replaced the valve spools and the tractor protection valve, but still does not work. No matter how long I hold the valve in it will not stay. The valve just gets really hard to push. But also once you push that supply knob you can no longer release the parking brake until you remove the red gladhand. Any help would be much appreciated.

 

Where the trailer connects to the truck.

 

Where would th

where would the quick release valve be located?

 

Does anyone have any more suggestions? I have replaced the spoils in the dash valves, the tractor protection valve and the quick release valves and I I still cannot get the trailer supply knob to stay in. I have tried it with the trailer unhooked and the gladhand plugged off and with the trailer attached but it made no difference. The trailer still would not stay supplied.