If you don't think it is safe to drive now (and I think you are right to be concerned) you could take it out of the truck, plug the lines, disassemble the master and inspect whether the cylinder is pitted. If it is pitted, you will probably have better luck getting it sleeved and rebuilt/remanufactured by someone like John Stuart
http://www.johnstuartpowerbrake.com/...-Sleeving.page in Stoney Creek. A new kit put into a pitted cylinder will wear and leak much sooner than new seals in a good bore. The master used in most of the Chev CMPs is a pattern that isn't readily available. Similar looking style exist, but not in the needed 1-1/4" size.
If only a kit is needed, this auction has ended on ebay but the seller regularly has CMP parts.
Based on a quite comprehensive Wagner Brake Catalog HU-400 dated September 1947, the master cylinder used on CMPs is probably a FD-4388 in "Wagner speak" based on shape, mounting pattern and bore. If this is correct, it is listed as taking a Wagner FC-3630 kit. The FC-3630 number in turn goes back to GM part number 603646, which is one of the repair kit numbers given in the C15A parts list. I don't know what the difference between this kit and the ones listed as superceding it are. GM part number 5450260 doesn't appear in the Wagner cross-Index, but it isn't one of the "unique to CMP" part numbers according to the parts list.
The FD-4388 seems to have been used in some GMC trucks late 30s to mid 40s.