My Knight was fitted with a breather by the Mac Read. He no longer makes them.
They were abused with the name of elephants trunks by the many attempted imitators, most of whom totally misunderstood the principles Mac used.
Mine has a specially cast ATD cover with a vertical bore into which is inserted (sealed) a vertical valve. I have never seen inside but believe it has a nylon ball which is blown upwards by crankcase pressure then drops to seal the end of the tube. This is enclosed in a sealed body which vents vertically into a rubber hose (hillman hunter heater hose)(alliteration helps to remember) this passes into a baffled box between the series D crash bars. Any oil knocked out of the air or condensed is sucked into a capilliary tube to the D breather cap when the pistons rise. My system then vents through the left crash bar. While it is possible to feel the engine breathing through this the exhaust is clean air. The oil consumption is negligible.
Mac's version for non faired bikes had a baffle chamber directly bolted to a D cap over the front exhaust valve. Someone else (I was told who by "plasticbeer" but instantly forgot) made some very attractive all stainless versions. These may still be available.
My Shadow has a system made by Phil Primmer which has a pipe from a non gauzed ATD cover, possibly from Kemps, through a large bore pipe to a reed valve. I think this relies on the length of pipe below the valve to let any relaesed oil vapour condense and drop back into the timing chest. I think Phil's intention was to fit a one way inlet valve with a Bunn filter, on the rear inlet valve cap, to reduce vacuum in the crankcase.
Cheers,