If it's a 51 3/4, does it have Huck or Bendix brakes? If they're Hucks, possibly the rear shoe is adjusted too tightly. Reversing out of the stall may just be enough to grab the shoe and bind. These brakes are not self adjusting, and have two adjusters per side, turnung in opposite directions.
Gooid luck!
Current fleet (subject to change w/o notice) \'49 GMC 3/4-Ton , 60 Austin Healey Frog-eye Sprite (some assembly required), 2011 Dodge Avenger, 2015 Jeep Cherokee. No, they don't all run. My other ride is a (B737)no, (B767)no, A320.... Update... was Embraer E190, now Boeing B787. Knowledge is a wonderful thing, but ignorance means you don't know you can't do something.
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