I had the same issue with mine last summer when I replaced all my master cylinder, airpack, brake lines and all my airlines. I ended up using a plumbing o-ring in place of the copper washer. It worked great. Best part is you don't have to crank the lid down as tight either. Then I added some other parts to allow me to attach my remote reservoir line.
The first pic shows how it originally was set up. What I didn't like was 2 -fold.
1. When attaching the remote reservoir line, it is a pain to bleed the air out of it.
2. to make it easier to bleed the brakes and not have to remove the reservoir.
The way I set up gives me a shut off valve. and a plug that can be removed to bleed the air out of the remote reservoir line or bleed the brakes.
This combined with the rubber o-ring has stopped all seeping or leaks from the top of the reservoir. Now everything stays dry and if it isn't...I know there is a problem.