Automotive History Capsule: Mechanical Power Brake Servo: Invented In 1919, RR Was Still Using It Up To 1978.

(first posted 12/27/2011; updated 10/26/2022)   The development of brakes pretty consistently lagged the development of engines and speed, which made most really old cars very hairy to drive. One of the breakthroughs was the mechanical brake servo, given that the modern vacuum servo was still some time in the offing, as well as hydraulic brakes themselves. How to stop the giant luxury cars of the teens? In 1919, the brilliant Marc Birkigt invented the solution for the superb Hispano Suiza H6, and Rolls Royce quickly licensed the technology. And kept using it well into the 1960s.

This diagram shows the first version of the earlier mechanical system, purely mechanical, where the foot pedal operated both the front and rear brakes, both assisted by the servo. The hand lever operating on the rear brakes is an emergency system. The picture after the jump shows the later version where the servo works on a hydraulic master cylinder. Here’s how the servo worked:

(click to enlarge to very large original size)

The servo is very similar to a clutch, and driven off the side of the transmission. Pressing on the brake pedal brings the other face of the servo disc in contact with the driven face, essentially like slipping a clutch. That generates a variable degree of force, which here is transmitted to the master cylinder. In mechanical brake cars, there were linkages running front and back to the brakes from the servo (the Hispano Suiza was one of the earlier adopters of four wheel brakes).

This diagram shows the later version, whereby the servo worked on two hydraulic master cylinders. Either way, the mechanical servo system worked quite well, and was reliable. Which probably explains why conservative RR kept it for the new Silver Cloud in 1955.

Update: This system was replaced by a fully hydraulic servo unit in 1966 for the Silver Ghost in 1966, but it was kept on the Phantom VI limousine all the way to 1978.