Air leak

I fixed something like that last week, I took it apart and bought some new O Rings at lowes

 

I just replaced this on my Pete trying to solve an air issue, $168 (new, not rebuilt) just for the part and that was just for the center section.
Can you isolate the point of the leak in the pic? It's name is, " anti compounding valve ".
I just saw the post about the o rings. BOL

 

Have you soaped it down to see where the leak is coming from?
I opened mine up OFF the truck on a bench after I replaced it. There's 4 Phillips bolts holding the top on, there's a spring inside so be holding it when you take the screws out. It's low tension but it could pop off inconveniently. The valve is pretty simple inside. I personally didn't want to try to fix myself and still have it leak so I replaced it.
That being said, if it's leaking at the sides where the air line adaptors bolt to the valve it might be as simple as tightening those bolts. Mine had an o ring that I reused on both sides,, no leaks.
Keep us posted.

 

That looks like a Haldex 4725001200X
Meritor WABCO Style ABS Valve Pack (ABS Rear Valve Package Unit)
The circle underneath that you describe is the exhaust. I would replace the whole unit.
The Big Green air Line on the bottom middle of the valve is the Supply Port.
This supply comes straight from the tank and is used on systems that are far away from the actual brake pedal valve (treadle valve) to allow them to apply quickly when you push the brake pedal so there is no delay in braking. What this means is that the foot valve air control is not the air that actually goes to the brake chamber. That air comes from the supply line itself but the foot valve is controlling that supply lines pressure on/off and in between. (It's a relay valve)
The Small green air line above the supply line is from the brake pedal itself.
The Two Valves on either side of the relay, the ones that have the black air lines hooked to them are called the modulator valves and are part of the ABS.
If you go up to the front of the truck and look at that valve there you will notice that it's nearly the same as the rear except that it doesn't have the big supply line coming from the tank that goes to it. It's modulated straight off the brake pedal itself.


http://www.haldex.com/en/North-Amer...-ABS-Valve-/Rear-Axle-Valve-Packs/4725001200X

 

Does it leak when the button is in or out? Button in Brakes pressed? Button pulled and brake pressed?

 

Just take it apart. If there's no pitting new O Rings and you should be fine. Those valves are pretty simple. A piston with o rings and a spring.