I'll go with Bills' idea too,'cos it makes sense,especially given the 'cap off' scenario, and the breather timing isn't all that ha?rd(?) and as I was planning to play with oil tank tomorrow,'cos it's all in bits- just had a head start. I blowed (blew) up the breather and it was a bit hard,(been sitting unattended for a while) but after poking a bit of wire up it came good. To do this in situ, (tank on), I would suggest blowing through the filler hole whilst poking wire up as it will purge gunk,(lizards, snakes,hornets,roadkill,maybe that other stuff you misplaced ) out downwards. Then it should all be sweet, and maybe add a vented old brake fluid bottle or similar at the bottom to catch oil vapour so it doesn't hit the road. cheers, good luck and have a nice day, duTch
Nb. For doing those tubie types of cleaning, you can use a pipe cleaner, like from a tobacco shop,or a gun cleaning brush -which come in a bunch of different gauges, but usually all short and need some ingenuity to extend them!!