Completely remove the parking brake cables at the clevis's at the parking brake cross bar. This will eliminate any interference from the parking brake cables.
This is the standard procedure for adjusting rear brakes.
Now readjust your brake shoes and test drive before you put the parking brake clevis's back on.
If the problem goes away, follow the directions for adjusting the parking brake cables for a '51 truck. That is described quite clearly in the Factory Shop Manual.
Carl


1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission