Abstract: The invention comprises an in-line cable insert for the parking brake cable system of a vehicle which maintains an already established tensile preload by eliminating slack in the cable system which would reduce the preload. The insert comprises a cylinder having an internal chamber, a piston sealed by a diaphragm therein, and a stem connected to the piston comprising one or more sets of ratchet teeth and pawls rotatably connected within the cylinder which engage the ratchet alternatively in half pitch increments. The chamber of the insert is connected to a vacuum source such as the intake manifold of a vehicle engine. The insert maintains the tension in the parking brake cable system when vacuum created in the chamber of the cylinder causes the piston to move downwardly, thereby tightening the brake cable, when the vehicle is started.