As Don alluded to, the brakes are asymmetrically designed down to having shoes of different friction coefficient for the front/rear pad on the same drum, and different strength of springs holding them. (This is off of Ross's video and the 51 service manual, but it's likely the same with yours.)
The unit is specifically designed so that the front shoe helps wedge itself against the drum, at the same time helping push the rear shoe against the drum, but this effect only works when braking against the car moving forward.
I would start by watching Ross's video and seeing if there is anything you missed.
https://www.youtube.com/watch?v=T1Zc3aW18Z4This Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=247767