The OE distributor was the same part for disc and drum and held no residual pressure or any other balance valving to either the front or rear. A replacement valve could, though I have not heard of one holding pressure to the fronts....

the OE residual valves were held inside the master cylinder. If you are using a 4wl drum master cylinder, that could be it. A disc/drum master might have one in the rear port.

Your first step was a good one. If you disconnected the rod and the brakes still drag, then that is not it.

Your next step is to loosen a fitting at the master and see if the brakes release. Also check to see if someone has installed inline residual check valves.

picture of typical drum brake master

picture of OE distribution block
this block contains a warning switch only.

inline residual valves

residual valve possibly located inside master:

to check for the presence of this valve, you would need the lines away from the master so you could insert a paprerclip or somesuch into the outlet port(s), and feel for something blocking the way, with spring tension.

swelled hoses or seals can also make the brakes drag. make sure no one poured anything in the master which does not belong.