Residual valves are only necessary when the master cylinder is installed lower than the wheel cylinders and/or calipers. I am guessing your MC is on the fire wall. The next thing I would look at is how you have it plumbed. How many outlets does the MC have and are you you do not have them crossed ie a the ports from the large chamber going to the front and rear and the ports from the small chamber going to the front and rear? It becomes important to know which MC chamber is designed for which set of brakes (front or rear) and what the plumbing passes through, ie proportioning valve(s) and residual valves. Brakes become much simplier when you know all the variables.
Unpredictable grabbing of brakes is indicative of oil or brake fluid leaking onto the drums and the old rear end can become suspect. I would remove both rear drums and look for signs of a leaking weel cylinder and a rear axle seal. If there is, take care of it and clean your drums and shoes well and try them again. If you do not see any signs of leaks, then check your rotors for leaks. If they are good, then return to the rear and check to see if your emergency brakes are releasing evenly and smoothly. If you have the old style cables and housings and they have not been replace in your ownership of the car, they can be suspect.