Don't know if this fits your problem or not, but I spent a frustrating several months trying to get brakes on my '46 two ton. An old timer told me the following as a way to isolate the problem and find out if the master cylinder was the culprit: He said to get three vise grip or gripping pliers and a supply of those sticks that you use to stir paint. He said to then clamp the flexible hoses leading to the wheel cylinders. Use pieces of the sticks to cushion the hoses so that you don't damage them when you clamp the hoses shut with the pliers. You need two for the front wheels and one will work for the back on the flexible hose located before the hose splits to feed the back two wheel cylinders. Then fill the master cyclinder with brake fluid and pump it, bleed if necessary to see if you can get pedal. If you cannot the master cylinder is bad.
I replaced mine with a rebuilt one which as I remember I was able to get from the usual suspects, Obsolete Chevy Parts, Jim Carter, American Classic, etc. Finally I was able to stop my old bolt!
