A method includes steps for controlling a pneumatic braking system of a trailer vehicle which is connected to a tow vehicle equipped with a hydraulic or pneumatic braking system. At the start of an actuation of the foot brake valve, an electrical switch is closed or opened, and a switching signal is transmitted to an electronic control unit as a braking start signal for an incipient braking process. A brake value sensor detects a brake value representative of the drivers current deceleration request and transmits the brake value to the electronic control unit as a brake value signal. The brake value sensor is used for determining the incipient braking process, and a backup valve is only deactivated by switching a redundancy valve from an open position to a blocking position if the brake value signal detected by the brake value sensor has reached or exceeded a predefined minimum signal value.