Chris J Dixon explained on 07/05/2018 :
AIUI, in reverse when the shoes touch the drum they move and are
then retracted from the drum, it would seem that this mechanism
is for some reason not working.

They use the fact that the overun brake is under compression and the
wheels are attempting to be be turned backward, to disengage the brake
shoes.

If the overrun brake is under compression, but the wheels are rotating
in the forward direction, then the brake shoes will work normally.

There is a full explanation here..

http://www.al-ko.co.uk/edit/files/ha...m-handbook.pdf