In my other car hobby this is usually attributed to a collapsed rubber hose going to the caliper or a malfunctioning master cylinder...Since both rears appear to be dragging it sounds like a master cylinder problem...
White Post Restorations (
www.whitepost.com) had to rebuild my master cylinder after I had 50 miles on a fresh restoration with the same symptons you've described...Excellent folks to deal with, have a stellar reputation and are considered a "staple" in the classic car hobby...They offer very fast turnarounds (usually less than a week) and are very price competitive...If nothing else you could discuss your issues with them and get another opinion...Ask to speak with Billy...
Hope this helps...Good Luck...
Let us know what you find out...
Later...
Rick