There is a ton of useful Triumph info on
https://www.triumphexp.com/forum/spitfire-and-gt6-forum.8/ The Spitfire / GT6 forum is particularly useful for brake and dash issues.
My drums dragged like they where out of round and I could not get them adjusted with a reasonable amount of pedal travel (they were too loose). I made all kinds of measurements and did all kinds of checks and finally noticed that the retaining pin / shaft and spring for the rear shoe was not sitting square. When installed it would always pull the shoe towards the brake adjuster and not allow the brake shoes to center. The rear shoe (non-parking brake) was in backwards.
Note that there is a notch in the shoe for the parking brake. The notch for the other shoe is located next to the brake adjuster. Putting in the brakes shoes correctly resolved my problems.
