Since it goes to the floor, no matter what else might need help, the first place to go is right to the rear brakes and make sure the shoes are adjusted correctly. Easy enough to do. Just jack it up, spin a wheel and, if it's not slightly dragging, adjust the shoes up by twiddling the star wheel through the access hole.
Some very early ones didn't have the hole in the backing plate, but your '72 should have them.
Then, when the brakes are adjusted properly (this is good for overall braking too of course, not just the parking brakes), then move on to the cable adjuster. If the condition of the cables is reasonable, adjust any excess slack out of the cable. There are procedures and measurements that the book would have, but you can get things close at least, by feel.
The cable assemblies can probably be purchased locally from any place that can order them. They're just parking brake cables after all. You could probably also find some donors at the local junkyard, but their condition would be questionable at best. Anything new enough to have perfectly good cables would have to have the same ends on it that our older ones have or they may not work properly.
I bought the longer cables from Wild Horses to compensate for the lifted suspension and they've worked great for years.
If all of this did anything, some of the pedal issues may have been overcome. But it really sounds like your springs and clippy thingies inside the mechanism need some TLC.
These things can get pretty rusty and gummed up over the years, so maybe all it needs is a good cleaning and re-lubing of the moving parts. If that's the case, and you get some tension into the cables through all that adjustment, then your ratcheting mechanism might just work as it's supposed to.
If not, well you can always replace it. But at least you gave it a good shot first.
Paul
