Your rear wheels locking up has nothing to do with the way it's plumbed. It's a mechanical problem. Like someone already mentioned, adjust your rear shoes, and while you're in there make sure everything is where it should be and that the shoes aren't mounted wrong. There is a leading and trailing shoe, and both have different length friction material. Also, check the contact surface of your drums and make sure there are no lips on the edges which could contact the shoes first. If in doubt, put new drums on it.
Edit; I just looked at your original picture of your master cylinder. Make sure that the lines going to the proportioning valve are correct. Sometimes the front reservoir is the secondary(like my truck), which means this would need to go to the rear brakes. Look at the picture of my Blazer below, and although difficult to see, the output lines cross each other before going into the proportioning valve.
Edit#2; I found a thread from this site showing the OP also has his lines crossed. Scroll down to the picture.
http://67-72chevytrucks.com/vboard/s...d.php?t=478154