Rear brakes lock 1st after MC change

[sleepyboy]

I have a 69 with vacuum assist brakes, disk front conversion, drum rear brakes. I drove over 2K miles with no problems. I changed to a chrome master cylinder from summit and put braided hoses in front. Now the pedal is more spongy and the rears lock up first. It drove around for about 2 miles then, the brakes got spongy. I parked it, the rear reservoir was 1/3 empty, but NO signs of fluid leak anywhere. I figured air in the line still. I have bled until I am blue in the face and then one more time. Everything worked fine before the change, but I am still using the same stock proportioning valve. Do i need an adjustable proportioning valve, or to just jump off the roof? The new MC is a little bigger in capacity, but it seems essentially the same. Any ideas guys?