[Capt_bj]

First thought would be to replace the spring in the drum as its function is to pull the shoes back when the pedal is released.

But, this spring is really not very strong and if the brakes are holding strong pressure then I suspect the shoes must be binding so I'd be looking for anyplace the shoe could be catching. Some thoughts are

the back plate is rusted and there's something there catching the shoe - take everything off, wire brush then paint with a heat resis' paint.

or

the drum is SO worn that the shoe is dropping into a groove and somehow is actually getting hung up.....solution is new drums and shoes

or - and this is really where you should start - is everything assembled correctly????? If not, there may be restricted movement just for that reason. First time I pulled my rear drums apart I found one side all screwed up assembly wise and it sure explained why I seemed to have no rear brakes (to the point the e-brake could not hold the car AT ALL)

digging deeper into brain cells suggests a blocked brake line that's allowing fluid to go thru under pressure but not return....a one way valve caused by blockage - but you saw nothing wierd when bleeding huh?

just random thoughts .... I'd take apart - de-rust and re-coat.