Hmm, ok I think I understand what you are describing about the energising effect, what I don't understand is how the drum brake system distinguishes direction. It would seem to me that the only difference in direction is which shoe is being applied first in the direction of rotation. In the case of reverse, the smaller shoe is first and with less surface area applied to the drum, is more easily overcome by putting the vehicle in reverse or by parking on a hill with it applied.
