This paper addresses the design and development of a Semi-Solid Metal (SSM) aluminum master cylinder for vehicle applications. This light weight, near-net-shape, fully dense aluminum master cylinder manufactured by semi-solid forming (SSF) methods is being supplied by Delphi for automotive and truck brake systems. Several million master cylinders have been put into service over the past three years with an outstanding field service record.The application illustrates the benefits of a supplier/customer joint development in the design and manufacture of lightweight, high quality, critical application parts at a cost competitive with other less reliable casting methods. One of the advantages of using an aluminum SSM forging in place of the traditional permanent mold casting is the improved dimensional control capability which makes possible a reduction in mass and final machining. The capability with SSM to forge what were previously machined features together with lower tooling costs, made possible with higher cycle rates and the need for fewer part cavities, result in lower cost than in the permanent mold process. Another advantage of the SSF process is the laminar flow of semi-solid material in die-fill, which results in lower porosity than liquid casting processes where turbulence can result in air entrapment and porosity with increased leak probability.Both process and design modification were required to accommodate product requirements for the vehicle and component interfaces, part performance, and manufacturability. Tailoring of alloy, semi-solid viscosity, tool lubricants, press dynamics, and part design illustrate the engineering collaboration that took place between customer and supplier. Through the application of robust development methods that involved part modifications engineered to improve processing capability and product performance and multivariable DOE testing, a process has been developed that meets the stringent, mass production product requirements for a critical automotive application.