Master cylinder cap/gasket

[Someday]

I just had my brakes checked by my mechanic and he says I need a new master cylinder cap - the gasket is bad. He was unable to find a local one.

After searching on line and this forum I am still trying to find a high quality gasket. I believe the cap is original and would like to keep it and just start with a new gasket.

Members of this forum, on another thread, suggested a Dorman part #42071. I checked the website and it states it's not the correct one for my vehicle!?

I am going to keep searching but any help would be greatly appreciated!

1971 C10 disc/drum (no brake booster)

Thanks!