Abstract: An apparatus for automatically sensing trailer sway and applying trailer brakes, thereby minimizing or eliminating the sway is provided. A hydraulic device, similar to a shock absorber with the addition of a pressure operated switch at each end of the cylinder, is mounted between the trailer A frame and the hitch head assembly. A sudden sway in either direction causes one of the switches to close and apply power to the electric trailer brakes thus minimizing or eliminating the sway and causing the car and trailer to "line up". The hydraulic device has an orifice in the piston or at each end of the cylinder so that normal turns made by the car and trailer do not cause sufficient pressure build up to cause either switch to operate. Similar automatic performance can be realized by the use of a gas filled bellows device.