Disk brake equipped 1966 Mustangs had a different master cly. than did a drum brake equipped Econoline van. Did you change the master cly? How about the push rod adjustment, is the master cly linkage free to allow return of the master cly? How about the rubber wheel hoses, I seen them completely shrunk closed inside resulting in fluid flow to operate the brake when compressed, but not release the brake when you let off the brake pedal. Is the master cly "too full"?

Just some things to consider.