Rear brakes lock 1st after MC change
 

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?

Re: Rear brakes lock 1st after MC change
 

You should switch to a disc/drum p-valve.
Did you clamp in the pin on front of the p-valve when you bleed the brakes? Also did you bench bleed the new master?
My personal preference is to get one of the latter masters w/plastic reservoir and cap and use a pressure bleeder.

Re: Rear brakes lock 1st after MC change
 

If the reservoir is going low,that fluid has to be going somewhere.I'd take the dust boots off the rear cylinders and see if they are leaking.If they are and it gets on the drum(s) then for sure the rears will grab etc. as I'm sure you know,and I'm sure you already checked this!
Other than that,double check that the drums and shoes are immaculately clean and adjusted right up until they drag a little.And re-bleed the MC.
Personally,I am going to an adjustable Prop valve simply because of the different loads I carry.But you DEFinately need to find where that fluid is going.