[ron skeen]

The threaded sleeve you are refering to is what I am calling a bung. I don't believe that tank is used in the brake system, ie not DOT controlled.

3/8" line has a volume of .110 cu in
2 x 1/4" .098 cu in = 3% less.

In your picture I see 6 x 1/4" lines comming out of the manifold that is supplied by that 3/8" line.
acording to my math? only 2 could be supplied with full volume @ any one time so I don't see a problem with a compromise.

I agree that this tank is a supply and nothing to do with the brakes. Do you have a tank in front of the drive axle pass side? This is usually fill by the 12v air compressor to run the toilet and doors. My 96 nor the 2004 do not have a tank in the blower bay and both are located in front of drive axle. I really don.t think that the volume would be that important on that line. My main supply tank is in the rear, front and rear brake tank front of coach with the aux 12v tank in front of axle. All are filled by all three compressors with the two brake tanks,( with one way valves)to were air can not go back into the system and is for brakes only.