[Doug I]

When holding on a reverse slope don't expect them to work equally as against holding on a forward slope. It should still hold though - just not as good in reverse. It's a function of the leading shoe principle that helps drum brakes work. The relationship between the diameter of the drum, diameter of the shoe and the location of the pivot means that when used the rotation of the drum tends to pull the shoes onto the drum. When going in reverse it tends to throw the shoes off - hence braking isn't quite as effective in reverse.

Check the cables are oiled and any pivots between the handle and the drums are working correctly. Everything else is as posted. Good luck