You want EPDM rubber, which you're not going to find at your local parts store. It's fully compatible with automobile brake fluid.
FWIW I'm the guy that provides the tool mentioned above to NPD. I've also got a number of prefab o-rings kits to rebuild the brake valves used on Fords.
This is the 8 dollar kit I ordered. https://www.musclecarresearch.com/valve-kit-wagner-drum-67-69
A little pricey with 7 dollars shipping. When you need it you pay.
That or 85 dollars for a new dist. block.
sah62
I will keep you in mind in the future.
Mike