Did this happen suddenly? Don't think you mentioned that.

-Is your parking brake working correctly? If not, your rear shoes are possibly out of adjustment.
-You have a hard pedal when pumped up so I doubt there is air in the lines. Plus you've ruled that out by bleeding the lines.
-It could be your master leaking internally, but I think you'd lose pressure gradually as you push on the brake pedal. One way to check is to remove the brake lines from the master and plug the inlets. You should now have a very hard pedal. If it leaks down slowly then you have a leaking master. Another thing is to check the linkage under the dash to the master. Everything connected and straight?
-Slightly low fluid in the disc brake side is normal as the pads wear. Nothing to worry about there. Doesn't sound like you're losing fluid.
-Run out on the discs can push the pads back to where you get low pedal, but your case seems extreme. Something to check though. Do you get vibration when you brake?
-Weak, bulging soft brake lines would cause a soft pedal, but again something to check.
-If it happened suddenly, it could be something amiss in the rear brake shoe linkage. That's where I would start, I think. Pull the drums, make sure everything is connected properly, maybe clean and lube the adjusters. Then manually adjust the shoes through the backing plate to just where you have a little drag. Try the brakes then.