So the reason why it must be installed after the distribution block is because the warning switch would think the brakes have failed because of the differences in brake pressure. I see in the picture that the valve piston slides to the "on" position when the Distribution block senses low pressure on the left side (which corresponds to the rear if I am not mistaken). In a car with front disc and rear drums, even if the proportioning valve (or reduction valve) was mounted after the distribution block for the rear brakes, once you applied the brakes and the pressure goes up wouldn't the block still sense that the front has different pressure than the rear? If this was the case, the warning light would turn off right when you release the brakes, but I'm curious why this would or wouldn't happen. Another way of putting my question is: doesn't the distribution block react to any changes in front to rear pressure (since the car was originally drum/drum)?