Power Disc conversion/ proportioning valve question

I would start by making sure the valve seems to reset. There used to be centering tools (for bleeding) that you could by, to hold them centered.

How hi/ lo is the emergency brake pedal when set? Maybe the rear shoes are out of adjustment.

I would also do a "check bleed" AKA just dribble a little from each (at least the rear) corners to see if you strike "air."

While you are at it make certain it is plumbed correctly. The rear end of the master (large section) goes to front brakes. Likewise, the prop valve is front/ rear sensitive.

See post 7 if yours looks like this one

https://www.forabodiesonly.com/mopar/threads/diagram-and-plumbing-the-proportioning-valve.262360/