Tom, the idea of back bleeding seems to be tough for me as i do not have the correct tools to do the job..but i'll have the brake lines inspected. The age of it - i have no idea, since i bought it from an owner who dont have a history of it..hope its not as old as the car (43 yrs old)..anyway, I'll get this changed as well...

The bleeders are to the top, no issues on this. And i'll try to find out where's the problem coming from by isolating the front & rear..

When you mention the pistons rolling back, could it be that the proportioning valve is not holding the pressure due to a faulty valve? I've checked for other sources on the internet and some of them are saying that a bad proportioning valve can cause either low brakes or wheel lockup.

Tools for back bleeding...............well it's somewhat embarassing to admit but there really are not tools involved; many years ago in total frustration over a brake overhaul on one of my beater cars I used a little oil pump can and forced the fluid into the system. I used a piece of small hose, stuffed one end on the bleeder valve and metal pump stem, zip tied the ends..........then, Pump and hold then close the bleeder let off the pump handle, open bleeder pump in more fluid close bleeder, repeat several times............this is a total road side hack mechanic method but it works. I now have a mighty-vac tool.

Finally I'd be willing to bet that the brake hose in the rear is still the original............because it's away from the drums up in near the driveshaft tunnel they seldom get looked at.

I agree with datoman, I had a similar problem. My problem was the rear brake adjusters, they were siezed.
It took three pumps of the brake pedal to move things back were they are supposed to be before I got a firm pedal.