In 1966 the mustang and Shelby had a very limited vacuum system. There were generally two vacuum lines. The first went from the intake manifold to the distributor, and the second (if equipped with an automatic transmission), went from the rear of the manifold to the automatic transmission’s modulator valve. There was a third larger vacuum line that went from the intake manifold to the power brake booster if the car had power brakes. For the most part, most cars had only one vacuum line, and that was the one to the distributor.