as far as i know, the function of a breather is to relieve the build up of gas in the crankcase area (which may be linked to the cam cover or wherever) caused by piston ring blow by.

an effective breather can be something as simple as a hole in the side of the crankcase, but that would piss oil everywhere so you have a separator to seperate the oil from the gas. it is that separation process that needs a little care to be most effective.

most engines have a basic system (like the 'kidney box' on a pinto/yb block) that are supposed to perform that function by making the mixture go through a twisty route or a gauze or whatever to try and separate the oil that then runs back down into the crankcase under gravity. as tony says, the std yb one is shit, as it pisses oil into the airbox, whereas 'clean' air should be returned there to be consumed by the engine. it's handy for keeping your k&n panel filter well oiled though if you fit the breather pipe before it

what i don't understand is why a lot of these systems that we see have 2 tanks one well designed one should be enough, surely? you would have the crankcase breather (and again, 1 should be enough if it's big enough) going to a vessel that has an outlet at the top for clean air to go back to the inlet and an outlet at the bottom for the oil to drain back to the sump. I can understand 2 tanks if one is a catch tank for the oil rather than having it returning to the sump.

and what's with the miniature filters? surely air is not sucked in through there, only exiting, so why the need for a filter.

maybe my understanding is completely wrong. could be a topic for one of stu's essays, or one of his articles now that he's a pro journo and has forgotten about us techies on here that love his free info