A method for operating a regenerative braking system of a vehicle by activating at least one motor employable in generator mode taking into consideration a first information with regard to a requested setpoint total braking torque and a second information with regard to an available generator braking torque which is maximally executable by the at least one motor employable in generator mode, if the setpoint total braking torque exceeds a threshold value and/or a piece of warning information regarding a transition to a backup level that is potentially imminent or has occurred of the regenerative braking system, even if the setpoint total braking torque is less than or equal to the maximally executable available generator braking torque, sufficient brake fluid is transferred via at least one hydraulic actuator to a wheel brake cylinder of the regenerative braking system that an air gap of the particular wheel brake cylinder is closed.