Jeesh Gimpy you are absolutely correct. That is on the air side of the compensator. I just assumed he was in the "unloader" for the air compressor.
My suggestion is to get a kit and rebuild the entire airpack. If you have this much corrosion in the air control part of the air pack, the entire assembly is probably in need of rebuilding. You will have to bleed the brakes afterwards, so lay in a gallon of silicone brake fluid.
It is vitally important that you identify which air pack you have. The older style has a "can" on the back that is 6 inches in diameter and at least a foot long or so.
The newer style has the same diameter can approximately, but it is significantly shorter, probably 5 inches and there is a very long cast boss extending back from the rear cover and bolts the air pack to the support bracket.
I guess what I am saying is that the long airpacks are about 20 inches or so long and the shorter ones, excluding the extension are about 12.
Hope this helps...
Wow Gimpy, good call on this one!!!!
RL