Rear Disks Staying Tight?

[72longbed]

I have converted my 72 K20 to rear disk brakes and still use the factory 3/4 ton master cylinder and prop. valve. I've been noticing that the rear rotors have been extreemly hot after even the shortest ride to the store or 1 or 2 miles anywhere. The front rotors are not hot at all when I check all four rotors. I have put the truck up on the lift and don't really notice a lot of 'drag' when rotating the rear axle without the truck running. (The 90WT gear lube that leaks from a bad axle seal on one side is actually smoking where it contacts the rotor!)

What I'm wondering is could the 2WD, drum/disk setup be putting undue pressure on the rear circuit of the brake system? If so, is there an easy way to fix this without replacing the master cylinder? I have heard of a disk/disk proportioning valve, but my understanding of prop. valves is that they do not hold line pressure on either circuit, but only are used to make sure that you still have front or rear brakes in the event of a failure in one circuit or the other?? Is this wrong?

Does anybody know what I can do here to fix this problem without replacing the M/C??

Thanks for your help,
72longbed