Atwood 85841 Surge Brake Actuator Replacement Parts - Master Cylinder Drum. Atwood Surge Break Actuators offer a unique design for superior braking that creates more pressure at the brakes than in-line actuator systems. The Lock-Out feature allows use with standard hydraulic brakes for backing up with no need for specific free-backing brakes. Breakaway device will set trailer brakes automatically should separation from tow vehicle occur. Master Cylinder Drum only.