This is Tie Down Engineering Part Number 70519K, a replacement Aluminum Master Cylinder for Tie Down Engineering Models 660 E and 750 E Disc Brake Surge Actuators. This Master Cylinder is to be used only with Disc Brakes. This Replacement master cylinder kit includes the Aluminum Master Cylinder, grease fitting rollers, master cylinder gasket, refill cap, and replacement nuts and bolts. The picture with this listing displays everything that is included in this kit. Master Cylinders are a very important part of your trailers disc brake system. They are mounted inside the actuator, and do the work of pushing the hydraulic brake fluid into your trailers brake lines, allow the disc brakes to work properly.