Prop valve fitting size

There are two ways to solve your problem.
Make your own brake line using this fitting:67-76 Mopar 1/2-20 Inverted Flare Tube Nut Fitting 3/16 Brake Line Tubing Tn09 | eBay
The other method would be to use an adapter so you can use your existing brake line. Edelmann #265301.