Disc Brake Hold Off & Prop Valves

In my case "I got lucky." Mine is 73/ 74 Demon duster factory up front, and a Lincoln Versailles disk brake rear axle. I just ran a nail through the residual valves, LOL, and "tried" the factory drum cylinder. No prop valve, and it works great

Depending on your tire size/ weight bias, and the disk setup front to rear, you may need a prop valve. There is NO WAY TO KNOW until you try it a bit