I was lucky with my 1969 T50 Chev, the previous owner had just replaced the master when I bought the truck. He told me that he got it at NAPA but that it was one of two in the country at the time. That was about 6 years ago.

I would suggest that you look for a company to rebuild the ones you have, usually done with a brass or stainless steel sleeve installed. Not cheap but it would be the last time it would have to be done.

Brake & Equipment Warehouse in Minneapolis (https://brakeandequipment.com/machine-shop/) is one place, there are many others.

Mac :{)

1962 K10 short step side, much modified for rally
1969 T50 fire truck, almost nos, needs a few things