Here is a rather simple schematic of a BW air brake system.

https://plus.google.com/10819770718...5163251346&oid=108197707187186692269

The illustration shows a MV3 Valve which was not available until about the 80's. Older trucks used a toggle type valve for the trailer and a push pull button for the spring brakes.


I would not recommend using Haldex because if you don't have the particular number you need, you will probably get the wrong thing. Haldex is aftermarket and most parts people are clueless as to what is what is what. Tractors have used a BW R12 or R14 valve for over 50 years. I order mine from Peterbilt in Minneapolis.
https://www.peterbiltparts.com/brak...0483bxw?gclid=CIPyxcmo8sQCFQwtaQodcTIAJw

The illustration is actually upside down. For the quick release for the spring brakes, all you need in back is a simple quick release valve.

http://www.finditparts.com/products...564&gclid=CJWTpMKp8sQCFQioaQodY0IAdg

Peterbilt is probably cheaper, but their server is very slow tonight.

On the dash the generic name for the valve is PP1. Here I think a Haldex can probably be bought cheaper. http://www.ryderfleetproducts.com/r...uct&gclid=CPP3652t8sQCFdYkgQodjKEAdw

Your truck may very well not have come from the factory with spring brakes. MGM invented them in 1956 or so and they were an option for quite a few years. My 1962 International DCOF 405 came from the factory with MGM spring brakes, but by then International had gotten way ahead of GMC in heavy trucks. Your spring brakes may well have been added later on by someone who didn't really understand what they were doing.