Time to pull the rear drums off and inspect/adjust them... At least that's where I'd start. Drum brakes don't work as well in reverse as they do in forward driving - nothing you can do about it, that's just the way they work. It sounds to me like the rear brakes are way out of adjustment, which will amplify the reverse no-workyness.
Obviously check all brake lines/parts for leaks, check master cylinder level, bleed the system if you find no leaks, and make sure your front pads are in good shape, and that a caliper hasn't locked up on you. Same deal in the rear - make sure the wheel cylinders aren't leaking, and that the shoes are in good shape, and that no springs or linkage are broken inside the drum.
