My ampmeter is pegging

Hi Guys,

I am having traouble finding a check valve for my Valiant. Does anyone know if power brakes were an option for the valiant signet in 1966? I am thinking someone might have installed a power booster and dual master cylinder and somepoint. I am looking for a 13/16 check valve and all I can find in a 1/2 inch valve.
Any clues?