Abstract: A trailer brake actuator and method of operation in which the actuator is interposed between a towing vehicle and a trailing vehicle and is operable upon a change in the distance between the vehicles to displace actuating fluid to the trailer brakes. The actuator is effective in both directions of relative movement between the vehicles, and in each direction of movement, the development of pressure above a predetermined minimum pressure or relative movement of the actuator parts beyond a predetermined amount will cause the pressure in the brake system to be released.