So the main brake line coming off the single circuit mc is 5/16" and uses 1/2-20 fittings. Looking at the pictures below the lines off the dual mc are 1/4" and use 7/16-24 fittings. If we use mc adapters that adapt to 1/2-20 inverted flare fittings, we wouldn't have to change the adapters on the air packs. If we reduce the mc fittings to 7/16-24, we would need to do the same to the inlet side of the air packs.
So for the mc to use 5/16" brake line, we would need adapters that go 9/16-18 to 1/2-20 IF, then 1/2-20 to 1/2-20 IF.
To use 1/4" brake line as the diagram suggests, we need mc adapters that go 9/16-18 to 7/16-24 and 1/2-20 to 7/16-24, then additional 7/16-24 adapters at the inlet side of the air packs. Not sure what the threads are at the air packs, I'll try and measure my spare in a few.
I found the adapters to go to 7/16" at the mc. Edelmann #258450 for the 1/2", #258403 for the 9/16". At least I think these would work. They are called master cylinder adapters in the description. If someone has better info or I need to be corrected, please let me/us know.
In the second picture there is a combination valve with the 2 lines in and 2 lines out. I had previously sourced this part as ac delco #15835212 but now looking for it, and any ac delco combo valve, they are unavailable at this time. This one was set up for the 7/16-24 IF brake lines. I will try to find another that has the 1/2-20 IF threads so if needed we could just use 5/16" brake lines all the way out.