See also in the REF section of the Sportsterpedia:

• Wet Sumping

• Example of Air Pulses Using a 1000cc Motor.
  

In the crankcase air system;
Crankcase air pressure is mainly generated by the up and down movement of the pistons.
Additional air pressure is created by blowby from the combustion chambers past the rings and into the crankcase.
Additional air can be created by other small air intake areas as well as from gasket leaks.
Crankcase pressure also initiates the splash and mist process (the movement of the pistons and flywheels splash oil around in the engine).
The piston motions create a pulsating blast of air pressure (push pull condition as each piston rises and falls).

The pressure in the crankcase is not from oil pump pressure.
Static oil pump pressure has already been dissipated by the time it reaches the crankcase.
(although it takes static oil pressure to get the oil from the pump to the crankcase)

Crankcase (CC) pressure will have an average and constant change in velocity.
Gravity oil (from the drain ports in the heads) returns to the crankcase.
Gravity oil (from the pushrod tubes) returns to the gearcase. Oil in the crankcase adds resistance to the air pressure generated (raising the pressure).

Role of the breather valve.
CC pressure both pushes to and sucks from the breather valve.
The breather valve allows for controlled air pressure to both leave and enter the engine to obtain what is referred to as a “slight running vacuum” in the crankcase.
If the breather valve isn't there;
The pressure generated from upstroke and downstroke would both push all the under piston air volume out of the engine and back into the engine.
(including any oil that was suspended with it. Also that would create thicker oil suspension and more oil blowing out of the engine but not returning.
See also, Example of Air Pulses Using a 1000cc Motor, in the sob documents at the top of this page.

The volume between positive and negative pressure decreases as RPM goes up.
The speed of the breather valve action is important as it has to keep up with RPM changes.

Piston / rod positions are relative to positive or negative pressure in the crankcase. Forget about valve opening for a second.

1. With the piston rising up,
   • The top of the piston is compressing or expelling air pressure in the cylinder but bottom of the piston is pulling in air (vacuum) into the crankcase.
2. With the piston descending down,
   • The top of the piston is pulling air (vacuum) in the cylinder but the bottom of the piston is both expelling air vacuum and turning it into air pressure.

These two conditions create pulsating air in the crankcase from vacuum pressure (negative) to air pressure (positive).
How crankcase pressure moves inside the engine does or doesn't contribute to wetsumping.

Intake vacuum is relative to piston / rod positions.
On carbed models with VOES:
Vacuum is created in the cylinders which pulls air (vacuum) from the carb.
This vacuum is tapped into and used to operate ignition timing through the VOES.
So a vacuum gauge plugged into the vacuum line at the carb does reflect the position of the two pistons.
I.E. the gauge moves when the pistons are on the way down and goes toward the resting position when the pistons are coming up.
(of course, you'd then have to take in accountancy if the intake or exhaust valve was open at the time)
So a gauge there would not reflect an absolute vacuum.
It would reflect more of an average vacuum based on running conditions with all the change variables working on the air pressure simultaneously.

Some time and tune older V-8s with a vacuum gauge instead of a timing light.
For each piston going up, there is a piston going down.
And intake vacuum stays (more) constant in the middle of the push / pull.
A vacuum gauge shows a more steady dynamic condition.
Likewise are the forces in the crankcase (crankcase pressure).

¹⁾

The rods \ piston action in the Sportster engine are close together (45° apart) and there is 360° around the flywheels.
If the rods were 180° apart instead, one piston would be going up at the same time the other is going down.
Just as in a V-8, crankcase pressure would be more equalized between them.

Since Sportster piston movement is not equalized, we get the potato, potato sound we all love but the equilibrium in the crankcase is off by design.
Take the PCV off the valve cover on the V-8 while running and it just smokes a little. Take the breather valve off the Sportster and oil pukes out.

Likewise, a vacuum gauge on the carb vacuum line will be erratic and not a very useful test. Hook up a vacuum gauge to it and the gauge bounces from vacuum to no vacuum pretty wildly at idle. Crankcase pressure is doing the same thing. But with the addition of the breather valve, it is a more controlled chaos in the bottom end.