I finally got the replacement master cylinder installed today. I had replaced the front master cylinder several years ago and the rear developed trouble after my annual brake bleeding this spring. I did an exchange with some seals from the old front master cylinder to keep it going while I could locate a new assembly.

Sticker shock! New master brake assemblies from Spartan cost over $700 each, even thru buss supply houses, they are in the $650 to $750 range, no one rebuilds these and there are no rebuild kits available. (at least in the Western half of the US!)

here is the write up:
Brake master cylinder replacement

hey im have the same problem with my master cylinder, and i cant find the part anywhere. do you have any ideas where i can find one at? what part # did you use when you got yours??

The industry that uses air over hydraulic brakes is serviced by Bendix and Wagner. New and replacement units in a variety of pressure applications do sell in the
$600 to $1500 range, BUT I have sent older industrial units(loaders, fork lifts, etc.) to Cardone Industries (www.cardone.com) and they can rebuild them, most of the time. I suspect your actual units were Bendix and carries the same p/n as current Wagner Industrial line. You might call Cardone and see if you can get them rebuilt.