Copper washers can be reused if the mating surface is not scared up. What you do to the is anneal them. Hang them on a piece of wire, heat with a propane until they just turn red, let cool. Annealing them makes them "dead soft" again. I do it all the time when working on my Classic Triumph MC and we used to do it on head gaskets and any other copper gaskets.

Make sure the banjo fitting or the mating surface on the plenum and bolt are not scared. That will cause leaks also.