Drivers side rear locks up really easily when I hit the brakes.

For all intensive purposes I have a brake system out of a 71-73 Dart, even though it is a 64.
Small pattern rims, dual master, single piston caliper disks in front, 10" drums rear. everything replaced new about 500 miles ago. Except rear drums, and proportioning valve. Was working beautifully, then I checked the fluid level and topped it off. It was about half full maybe a little more.

Now when I hit the brakes the drivers side rear locks up. The car seems to stop ok but I can't drive it like this. It really freaks people out when I come up behind them, not to mention passengers in my car.

Is this an adjustment or did I get some crap in the proportioning valve or master?