Oil breather/leaks

Hi all - my 920 Mk2A is now run in (1500 miles) and runs a treat - one issue though:-

I removed the old air filter system a long time ago and fitted a single Amal Mk2 which I was never happy with - I have subsequently refitted 2 x Amal Mk1s - the bike starts and runs much better. Obviously the old engine breather system went when I fitted the single carb - I fitted a one-way 'clack' valve in the breather pipe from the crankcase to the oil tank - this has improved oil-leak problems (the engine is 100% oil-tight). I ran a pipe from the oil tank filler neck to the rear of the bike as the main breather - this works OK but I do tend to get a fair bit of oil dripping out of this when the bike is stopped after a run - the leakage does seem to vary from run-to-run. I gather that the the oil level in the tank has a lot to do with this and I try to not fill the tank above the 'low' mark on the dipstick which seems to help but I'm perplexed as to how so much oil finds its way up to the top of the tank (i.e. the filler neck). I also noticed that the filler-cap seal seems to be worn - I cannot make out what the material is - is it metal or rubber etc. - some oil seems to weep down the right hand front of the rear mudguard - I cannot see where it is coming from but suspect it could be the the filler cap - all other joints etc. seem bone-dry.

Any ideas ?

Cheers.