Air tank valve identification and equivalent replacement part.

Does anyone know where I can get a replacement air brake valve for my Acco 1850D. The valve hangs off the airtank and has one line that goes back to the foot pedal brake control valve.
I am thinking it is some sort of Quick release valve but this one has a side adjustment screw with a heavy spring behind it that a QRV does not seem to have.
Any help appreciated.

Is your truck full air or air over hydraulic? (I am guessing here) If the latter it is probably a pressure control valve.

Ping someone like MaxiParts and see what they say

I'm afraid i only have books up to the C series, and i can't seem to locate it in them, i was hoping to find a parts description. Would it be an adjustable valve that only supplies air to the treadle valve once the tank reaches a certain preset pressure?

Can you strip it down and repair it?

Truck is full air.
I have stripped it but the rubber bits inside are no good.
I will send an email off to Maxiparts to see what they think.
Could be something associated with the treadle valve, going to have to read up on airbrakes a bit.
Thanks guys.

As a follow up that particular valve is called a Holdback Valve and is made by Goyen Controls Milperra NSW. Their part number is HBV-1/4-R and it seems they do a repair kit also. I never contacted Goyen but ended up getting a Pacific Air Control brand where they call it a Pressure Protection valve that does the job under part number ABCN15759BA. The Goyen valve is apparently $150+ as apposed to the later which is about $30