A method and system for operating a combustion engine brake especially for a heavy vehicle like a truck or a bus is disclosed. By detecting a brake torque value which is delivered by the combustion engine brake for braking the vehicle, and, if the amount of the difference between a demanded brake torque value and the delivered brake torque value exceeds a predetermined first limit value, by operating the combustion engine brake by means of at least one additional or alternative second operating parameter which is selected in order to create a corrected brake torque value, a considerably improved accuracy, efficiency and/or reliability of the combustion engine brake can be obtained.