UncleD The crankcase in effect breathes via a keyhole in the timing side casting just above the oil pump. The keyhole also allows oil to drain from the oil pump cavity back to the crankcase. There is free air passage between the crankcase and the area around the timed breather, so any change in crankcase pressure will affect the pressure within the timing covers, relieved when the breather opens.
The timed breather bush is sealed against the camshaft drive gear by a cork washer, available in various thicknesses, (or make your own) as outlined by esteemed member bikerbob, of a thickness which eliminates end float on the breather bush and camshaft without the bush binding too tightly against the inner face of the timing case. The cork should be just lightly compressed enough to give an air tight seal.
The rotating breather bush opens a passage which runs across the back of the engine, to exit amongst the dirt and muck above the gearbox sprocket. So if the cork appears in order, check this breather air passage is clear by blowing through with the air line. The inner cover will have to come off, so only worth doing if you suspect it to be obstructed or blocked, or the existing cork is allowing float on the breather bush. In theory you could seal the airline against the bush and rotate the engine in small increments until the breather opens and listen for leaks or free airflow. A bit tedious, but saves dismantling. The system should hold pressure until the breather opens.
What you describe is normal, as the breather only opens for a very short time, and a twin cylinder engine with pistons moving in unison is a very effective air pump, hence the rise and fall in crankcase pressure until the breather opens.
If you decide to remove both covers, check for slop in the idler gear bushes and that the timing marks on the three gears are aligned correctly. The small cork washer behind the large dynamo drive sprocket crudely controls the idler gear end float, by being sandwiched between the back of the sprocket and the front face of the inner cover.
Swarfy.