The adjustable proportioning valve is used to tune brakes. The valve goes in the back brake line, affecting only the rear brakes, and reduces pressure to prevent the tires from locking up. If the car is running wide tires in the back and narrow tires in the front, there is now more surface area in the back causing the rear drums to do all the stopping. This valve reduces pressure to even out the brakes and is ideal for use with the distribution block shown earlier. The distribution block provides equal pressure to all four wheels, and again this valve goes in the rear line to reduce the pressure to the rear brakes. Ideally, the front brakes should be getting 70 percent and the rears 30 percent. On four-wheel disc applications 50/50 is normal.