A method of controlling a transmission brake that is actuated by inlet and outlet valves. For determining when to transmit a disengagement signal for opening the outlet valve, the current input and output speeds are determined and the respective gradients of these speeds are calculated. Variation of the input speed, during the disengagement of the brake, is determined as a function of the input gradient, the optimum time for reaching the synchronous speed, during brake disengagement, is determined from the input speed variation as a function of the output gradient, and the time interval until transmission of the disengagement signal is determined as a function of the current input and output speeds and their gradients, by a back-calculation from the time when the synchronous speed is reached, taking a disengagement dead time of the brake, between transmitting the disengagement signal and start of disengagement, into account.