I have a 1998 Econoline 5.4 with 280,000 miles on it. I love it. However, the rear drum brakes work great after being manually adjusted but they lose that adjustment in a few hundred miles of driving. I've done everything EXCEPT replace the parking brake cables. Could that be the problem?
Did you replace the hardware the last time you replaced the shoes, particularly the star gear, tensioner cable and adjustment lever? If not, the springs are getting weak. Did you use brake grease on the star gear and the guide that goes in the other shoe?
My '99 Ranger pretty much requires you to use the parking brake to keep the adjusters working. The manual also says to intermittently reverse and brake relatively aggressively to force them to adjust if you don't use the parking brake often. I religiously use my parking brake, and the brake hasn't loosened up from wear in the 30k miles I've driven since tearing the drums apart.
They should self adjust when you go in reverse. Maybe the star wheel adjusting screws are installed backwards, or the lever and spring mechanism are crudded up and need to be cleaned and properly lubricated.
