drum brakes are not designed to hold a load well in the reverse direction which is why they have been phasing out on cars for years. They also hold heat which is another reason. It is also why when you engage your trailer brakes your tow vehicle can drive through the brakes easier in reverse.
The way the self adjuster works (and I do have it on mine) is by controlling the amount that the brake pads shift on the pivot studs until it bites into the drum. With the drum off you can physically wobble the whole mechanism around. In the fwd direction the attack angle of the pads minimizes the wobble so the brakes grab quickly. Unfortuantly this geometry has the opposite effect in the reversing direction and allows the pads to move further as the pads wear down (you will notice that the pads are mostly worn on one end, thats the fwd contact surface). On self adjusting brakes like mine, this movement actuates a little lever that spins the star adjusting wheel a click or 2 so the next time the pads don't move quite as far. The adjustment of the star stud changes the trailing edge contact/approach angle to the drum. Effectivly as I back up and trip the brakes, each click of the star stud wheel will increase the amount of the pad in contact with the drum and decrease the wobble distancee until it bites. this ends up greatly improving the fwd stopping distance which the brake is designed to be biased towards anyhow.
2010 Surveyor SV 291
06 F250 4x4 crew cab PSD
1 DW, 1 purebred mutt, 1 ninja beagle
Nights camped in 2011: 19