These are used for cars with VERY high lift Cams, that do not produce enough Vacume for the Power Brakes.
I have reduced my Cam from .621 lift to .515 lift as I am converting from a Drag car to a street car (73 Mach1), and I don't need this anymore.
I don't know much about them, but I know this one works perfect as I was using it until recently.
$ 25.00
you pick up (no shipping)