When the engineer uses the automatic brake valve to reduce the pressure in the brake pipe, he dosen't actually reduce the brake pipe directly. Rather he reduces the pressure in the equalizing reservoir, and the attached equalizing piston opens the discharge to reduce brake pipe pressure accordingly. This does two things: 1) the eq pistion closes the discharge valve slowly, which avoids pressure surges that can release the brakes on the front part of the train, and 2) allows the engineer to manipulate the brake valve for a given amount of brake application in precisely the same way no matter how long the train.

There are some automatic brake valves which do reduce the brake pipe directly, but these are either antiques or rather rare special types.