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.