Tim,
Before moving into any other diagnosis (or speculation),
check your toe-board clearance adjustment (for the master cylinder), and then
adjust the brake shoes. Note that the 1951 instructions state to adjust to tighten until you get a drag; then back off 14 "notches".
On my 1/2 ton 54, I tighten until the brakes shoes are really tight (to "seat" the shoes). Then, I start backing off until there is no drag. Then, I back off 7 notches (7 notches is what is called-for in the 1954 truck Shop Manual).
