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Residual Valves: Residual valves are pressure valve use to retain pressure in the lines. The most common use is on a hotrod when there is a floor mounted brake pedal and master cylinder. Mounting the master cylinder (M/C) below the floor positions it below the calipers. Gravity will cause the fluid to flow away from the calipers. The residual valve will retain pressure within the lines. (i.e. 2 pounds residual valve will retain 2 pounds of pressure, 10 pound will retain 10 pounds.) Drum brake master cylinders have residual valve(s) built into the master cylinder. This is needed to maintain pressure against the cup seals in the wheel cylinders. If you are using a disc brake master cylinder or after market you will need to install a 10 pound residual valve for the drum brakes. Do not install a residual valve if your master cylinder already has one in it. This will cause the brakes to lock up after the second application to the brake pedal.
Distribution Blocks or Combination Valves: One of the biggest misconceptions is the distribution block or combination valve. Almost every factory car has one. This usually serves as a metering block to adjust the proportioning to the rear brakes, as a "T" fitting for your front left and right front brake lines and brake light warning switch. What people fail to understand is that each car is "engineering" for a specific distribution block based on weight, braking characteristics and tires. So generally most factory cars have different blocks.
Save yourself some headaches install a adjustable proportional valve in the rear brake lines.