Random air line leaking when press brakes... Anyone know what this is?

VIN - 869782

I stopped at a KW dealer a couple states away when i was on the road, just got back. The picture is of the "brake service manifold" apparently. The guys didnt know what the word "schematic" meant so i dont know if what they said is very credible. One guy said most of the trucks all have the lines ran for accessories and if the truck didnt come with it they are capped and a cap could have fell off??? Another guy seems to think it is the MV3 brake valve in the dash. He did find a diagram of the air lines for the brake system, though he had no idea what any of them were it shows one line going into that manifold and then out (probably to rear spring brakes), and then another one just going to it...which is orange and doesnt go out or anywhere. He said the brake pedal is back feeding the MV3 valve and then they wanted to sell me one for $365.

To the other questions, I bought this truck in May, it was in great driving condition, drove it home 300 miles no issues. Everything worked and it sat for 3 months (cut in half, stretched, flatbed put on) No, nothing was touched up by this... I did everything myself so I know. As im completing this project ive been inspecting it more in detail so it will pass a DOT inspection, first time i moved it in months and heard this air leak when the brakes are pressed.

My question..... Does the MV3 valve have anything to do with the brake foot valve? The MV3 valve is the park/trailer brake release on the dash which I thought was completely separate from the service brakes isolated from each other???? I will see if it leaks when the park brake is on and when it is off, if there is a difference.
Thanks.

 

Yes i had someone pushing the brake pedal while i had this in my hand. It is definitely the leak, and it bleeds the air tank down fast. There is a brake application gauge on the dash and it does work. Could be an accessory line like you say, cap could have fell off? Dealer guy did mention that. But what accessory would be activated by brake pedal?

 

Thank you so much!! The truck did have an air slide 5th wheel on it, no line was going to it though. The fact air comes out of it when you press the brake I would think rules out a 5th wheel slide.. I will pull apart the dash today and see whats in there. And try to trace it better. Also the one diagram shows a 1/4" orange going to the rear service manifold for a dual air pressure gauge, mine only has a single pressure gauge.... But I would think that would be the source of the air where the gauge gets its pressure, I have the opposite, the line blowing out pressure. Its definitely brake related though.