A combination valve and a proportioning valve are not the same.. A proportioning valve goes in the rear brake lines to limit the amount of pressure to the rear wheels...

As pictured, the combination valve has a proportioning valve installed in the rear wheel brake line...

The two bowl valve will not close off a system when a line breaks, the bowl with the broken line simply runs out of brake fluid and all the braking is done with the functioning end. A combination valve will close off the line to the end with the broken line, allowing both bowls to supply brake fluid to the end still functioning.

Residual valves are designed to maintain a small amount of pressure on a brake caliper or wheel cylinder so that it takes less pedal travel and less time to apply the brakes. A residual valve is also necessary in a system whenver the master cylinder is floor mounted and below the level of the wheel cylinders or calipers. It then also acts as a check valve to prevent fluid drain back to the master cylinder...

To say they are not necessary because you don't plan on trying to stop on a dime is much the same as saying you don't need them because you don't plan on being in an accident!!!!! All of the components in a brake system are necessary pieces and need to be installed in the system. The braking system is not a good place to take shortcuts or try to save a buck or two.....