losing brakes

Imho. Anytime the pedal goes to the floor there is a lot of fluid going somewhere.

If it only happens when backing I would first check that the rear brakes are the correct diameter.

Without adjusting the brakes see if you can remove the rear drum.
Then measure the diameter of the brake shoes. And the diameter of the drum.

My thinking is in reverse the self energizing action of the drum brakes is not as good as in foward. Maybe the shoe diameter is too small and the displacement of the cylinders exceeds the stroke of the MC.