What is inside an oil filler/breather cap?

You could try using methanol (methyl alcohol) it will loosen debris, remove varnish build up, not destroy the filter media, and it will dry without needing a water rinse.
Alternatively you could use isopropyl alcohol (rubbing alcohol is too dilute) but it takes a bigger amount to do the same job as methanol.

You could shorten they drying time by using gentle compressed air.