Air valve looks Bendix, kinda like a R-6...that's a place to start anyway. Looking again, the bottom is not R-6 as R-6 has four delivery ports on bottom, but the domed top is similar.
If the valve only has the four lines, probably the large one (shiney brass) is supply from an air tank, the smaller one is signal (from your brake pedal) and of course the bottom two are delivery...no problem with the compounding issue. I am assuming this is a service brake relay valve...yes?
I guess I shoulda stuck with my first impression! I think the RE-6 has the four ports, two 1/4" & two 3/8"