[WE7X]

Lewis,
Thank you for that input, and I agree with the diameter vs. fluid transfer issue.
I found a Wagner SC45289 listed at Rock Auto; as fitting a '64 or '65 3000 series, and the picture looks correct. The alternate item ( Dorman CS36124 ) has the slightly larger bore, and the input fitting location is on the top, vs. on the bottom in my particular setup.
I then found a source one at Centric, and a bit less expensive than Rock Auto.
It has been ordered.
I would have been fine with having the local shop putting a sleeve my old cylinder (which they would do for the same cost), but we had not yet found a replacement piston. I am sure both of my original pistons will be in pretty bad shape if I can even get one of them out in one piece.
Now off to the local NAPA for some new supply ( remote reservoir setup in this rear engine chassis) ) and return hoses, and to get a new pressure hose fabricated.
It seems to very rare for any place to have any parts listings or information on anything bigger than a 4000 series chassis in a roughly 50 year old GMC. Rod J
Issaquah, WA