I would guess the 1939 1/2 ton panel truck uses the same master cylinder as a 1939 1/2 ton truck.
The Master Parts List says 1 inch diameter master cylinder 1938-1946 for a 1/2 ton.
Cast number 5450256. Part number 5454575 correction 5454576. Is this what you have now?

Last edited by buoymaker; 06/12/2022 5:51 PM.
"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."