Tie Down 750E Drum Brake Surge Actuator - Single Axle The Tie Down Model 750E Drum Brake Surge Actuator works by the surge or push of the trailer toward the tow vehicle. This automatically synchronizes the trailer brakes with the tow vehicle axle brakes. When the trailer pushes against the tow vehicle, the actuator telescopes together and applies the force to the master cylinder, supplying hydraulic pressure to the brakes. The built in dampening shock absorber retards the telescoping shock again