Need help with back bike brakes

Hey, I have a Schwinn Mountain bike (I know booooo me) and am having issues with the brakes not really working in the back.

Essentially this is what I'm stuck with:

I pull the lever as far as a physically can and it only goes about halfwaym by then the calipers are gripping the rim but I can still, while holding the brakes still, easily move the wheel with my other hand. I've replaces the pads, checked for cable damage/mangling, and everything.ni really just can't figure out what's wrong with it.