Master cylinder lid seal 68 dart

Hi guys,my 68 dart is leaking brake fluid from the master cylinder cover. I pulled it off and the seal is still there,I cleaned it off,reinstalled it making sure it was positioned correctly and it still leaks. Since there’s nothing wrong with the master cylinder I’d hate to replace it just for the seal but so far none of the parts places I could find carry the seal.
Here’s a photo of the master I have. You can see the fluid leaking under it

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