My knowledge is nothing compared to what Martin can offer, so i'll let you guys carry on but thought i'd chime in as i've just finished rebuilding the rear drums on my '79.
First thing i'd say is about the cable the goes under the car and splits in 2. This is for the parking brake only, so only works when you put your foot down on the parking brake. You can jack the car up, start the car, put it in drive and then when you apply the parking brake, see what happens. If you have rear drums, i wrote an express run through below, so once you've done all that, come back and then you can adjust the parking brake if needs be, by turning the nut by the divider (if the parking brake isn't putting enough pressure down, once you know the system is all good).
Next thing is about the foot to the floor thing... sounds like the system could use a bleed (once the rear problem is fixed of course). Do you know how to bleed a system? A few good youtube vids about it, in a nutshell you open a bleeder, have a friend squeeze brakes slowly and then close the bleeder and move around the system until it's all gone.
Next one is more of a question... do you have drums or discs on the rear?
If it's drums, then read on, if not, ignore the rest
If it's drums, then jack the car up, take the wheels off and pull the drum off. Should come off with a bit of a tug. First thing to check is the brake shoes - are they worn? I have a feeling that one side has worn more than the other. The solution is of course new shoes. Let's say that the the shoes are all about the same thickness, then there is a way to balance the rears within themselves (you are correct - one brake fluid line for the front, one for the back). At the bottom of the drum brake assembly, there is a little bullet looking device, with teeth on it. You can rotate this so that it moves the shoes farther out or more inwards. What you would do is on the side that isn't braking enough (provided that there is still meat on the shoes and they aren't completely worn) is turn the cog a bit, try reinstall the drum, if it goes on easily, it's probably not far enough, so keep turning until you feel a bit of resistance when putting the drum on. All of that, is assuming that the drums and shoes are all good and just need a bit of a tune. Once you do that, bleed the system and you could be all good. But of course, could be more complex, that's just a starting point.
I can do you a run through video tomorrow if you need?
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