1972 K20 Brakes - Need Help

[1972RedNeck]

Over the past couple of weeks, my brakes have been acting up a bit. Every now and then, I would get a hard pedal and the brake light in the dash would come on. If I came to a stop and firmly reapplied the brakes, the light would go out. About this time, the rear brakes started to be over zealous as well trying to lock up with light pedal application. Furthermore, whenever I checked the brake fluid, the front brake reservoir (towards the rear of vehicle) would be completely full and running over and the rear brake reservoir (towards the front of vehicle) would be low. Fill the rear brake reservoir and empty the front and in a week or so, the front brake reservoir was over full and the rear was low.

Yesterday, I stopped at Napa and grabbed a new master cylinder, bench bled, installed, and tried to bleed the brakes. I could not get the rear to bleed out - I keep getting air bubbles. I moved to front and they bled out perfect and then went to the back again. Ran another pint of brake fluid through it trying to bleed the rear and am still getting air. Got it to where I was just getting a bubble or two every other pedal stroke and said the heck with it and took it for a spin.

My brakes are acting EXACTLY the same as before. The rear tires lock up with little pedal effort except for every now and then when I get a hard pedal and the "brake" light comes on. Haven't driven it enough to tell if the reservoirs are still swapping fluid or not.

What gives? Why can't I get the air bled out? Why was my M/C swapping brake fluid between reservoirs in the first place? Why did a new M/C not change anything? Could the proportioning valve be bad? It doesn't leak...

Anyways, I'm out of ideas and could use some help.