I don't know if others got the same failure I did when clicking the links above. I found
http://www.mapleleafup.net/forums/sh...ad.php?t=11273 and
http://www.mapleleafup.net/forums/sh...ad.php?t=13772 to work better.
Just guessing, but since I don't believe the GMC used air brakes on many (any?) of its trucks in this weight range (but they did used vacuum assisted hydraulic brakes) there might be a parallel to the use of vacuun controlled brakes for trailers similar to those that could be found on CMPs. On the CMP application, there was a cylindrical vacuum reserve tank so a similar tank may have been mounted on the GMCs.