I just took the stock one off the slant six in my 73 dart and set it on my workbench to remind me to post a question just like this. LOL
I would think that dousing it with carb cleaner should work, it may take a couple of cans but it should work.
I just figured that $2.99 a can is still cheaper than trying ebay for a NOS breather seeing as how NOS Ebody ashtrays are worth 400 bucks or more.