Chris:
Ford did not install hydraulic brakes in any vehicle until the 1939 model year. Your 38 truck almost undoubtedly has '39 up (to '47 half-ton, '48 passenger).
Although two different wheel cylinder sets were used in that time frame, most vendors only sell the later ones which are slightly better.
Check to make sure that your brakes look like:
39 to 42 brakes
or
46 to 48 brakes
(note the difference at the bottom of the backing plate where the shoes pivot), then order whatever parts you need.
Mike
