The design and development of Mark II GT brake system within the parameters dictated by the Mark I chassis presented many problems. The Mark II GT with its larger 427 cubic inch engine had more weight and much higher performance than the Mark I. Space limitations of the carryover wheels and suspension imposed a severe handicap on individual brake component design. This was compounded by shortening the normal one year development time to a three month period.Part I of this paper is devoted to the consideration of factors which control the design of a brake. The concept of kinetic energy and its effects on brake performance is reviewed briefly. Use of the ventilated rotor design is explained for applications where severe heat is a problem, as in the case of the Mark II GT.The development of the brake system from the 24 hour Daytona endurance race to the Le Mans Grand Prix race is reviewed. And unique rotor problems resulting from the various energy loads experienced at Daytona, Sebring and Le Mans are analyzed.In Part II the brake dynamometer, its automatic programmer and the logic of race simulation duty cycle are described.Use of Ford's new Reliability Laboratory brake dynamometer for screening of potential rotor designs is explained. In the screening process, dynamometer results proved that significant brake development work can be performed in a laboratory where a race can be simulated under carefully controlled conditions.