How would it do that? PCV valve is not a 1-way valve, it's always open (the area depends on engine vacuum).

I thought it was. :confused:

It's only a one way valve with the assist from vacuum to close it.
Without vacuum, it's a loose open fit to atmosphere.
Of course, pressure exerted from outside - in can also force it closed to a one way valve.

Thanks for that; interesting.

Transpose that thought to someone opening the oil cap with the engine shut down.
At rest, the system inside should be at atmosphere.
Else, the breather is stuck closed and not allowing pressure to equalize.

Here we go again. Vacuum and pressure are two sides of the same coin. PCV valves allow one way flow, out from the crankcase. There's air flow because there's a pressure differential. Some valves have more "preload" than others needing more pressure differential before they allow flow.

Vacuum or pressure depends on your perspective. If you were in the crankcase while the pistons are coming down you'd see atmospheric pressure outside (relatively) as a vacuum.

It's always open.

I see what you are getting at.
But the source of air movement usually gets top billing.

In consideration of stop motion of a piston raised from the bottom to the top and vise versa.

The term “vacuum” is used to describe the zone of pressure below atmospheric pressure.
Vacuum is a negative gauge pressure, usually referenced to the existing standard barometric pressure where the equipment will operate.

If you are looking from inside out and watching pressure go out the breather hose, it could be perceived as vacuum (or other source) from the atmosphere sucking the air from the engine. \\
However, atmosphere cannot move without opposing forces affecting it.

Piston downstroke, by default, is the catalyst that moves air out of the engine starting when the piston is fully topped out so we label the offending party as positive pressure.
http://www.sportsterpedia.com/lib/ex...hippysmack.jpg

Piston upstroke, by default, is the catalyst that pulls air into the engine starting when the piston is fully bottomed out.
http://www.sportsterpedia.com/lib/ex...hippysmack.jpg

This is a Ford F-150 PCV valve.
The source that opens and closes it is different than the Sportster but the operation of it is expressed.
Notice the notations.
The vacuum line is installed on the top nipple and the hose is ran from the carb / intake (negative pressure is the source affecting the valve).
The bottom portion is installed into the valve cover.
(the cover over the breathing system in this instance).

At idle, it shows high intake vacuum and low CC pressure flow.
At that point, the pressure is pulling on top of the disc.
There is still however some portion of flow thru the PCV at idle since the top disc does not fully seal to the top of the housing.
The disc rises toward the top position due to higher intake vacuum pressure.

As speed increases, vacuum lowers and the disc does also.
However, at acceleration there is lower intake vacuum and higher CC pressure flow (higher windage in the CC).

Only during a backfire through the intake / carb does the valve fully close (positive pressure is the source affecting the valve).
At that point, the pressure is pushing on top of the disc.

Now notice the location of the spring. It is below the disc, not attached to it.
Therefore, the disc will not fully close off CC pressure unless there is a positive force pushing the disc from above.
With the engine shut down, there is no force to close the disc down.
This will equalize pressure between atmosphere and the CC just as Sporty breathers do.
It takes a pressure source (either positive or negative) to turn it into anything more than a decoration.

https://www.f150forum.com/attachment..._operation.jpg

edit:
I've taken the vacuum hose from the PCV off my Chevy just to see what would happen.
The disc floats at idle but doesn't fully open since the main source of pressure was disconnected.
This does change CC pressure output.
The PCV emits a little smoke presumably because the pressure balance has been changed.
(like when I pulled my oil cap from my bike and the engine stumbled with the one umbrella not up to par)
At that point, CC pressure is pushing the disc open slightly.
But the pressure is not enough to fully lift the disc.

I've also pulled the PCV, cleaned it off and sucked on engine side (a way to test if it closes as it should).
The valve closes when I pull a vacuum on it but it doesn't start out closed.
Every time I pull vacuum, it snaps closed.
This means there is air between the valve and seat that I remove with my mouth and the 'snap' happens when the valve closes.
I'll let go then suck again and it smaps again. It doesn't close on it's own.

In the Sportster engine, the source for both positive and negative pressure is the pistons instead of the intake as in autos.

In L82-up engines, the umbrella(s) by default should float as the RPM goes up.
The pistons speed is based on throttle position while the umbrella(s) speed is based on construction material and condition.

In 76< engines, the timed breather spins at 1/2 speed of the engine (albeit the racing 1/4 speed pump) and I believe oil scavenging and crankcase pressure is sent through the same passages to the oil pump where air pressure is released by the rotating slots in the metal breather valve itself.
So construction material stays constant with the variant being wear and poor valve timing.
I sure wish someone with an ironhead had access to a slack tube and was willing to do some testing on it. :)

Here's one for a Ford 302 engine. Looks kinda funky in operation.
But these still require a good amount of vacuum to operate properly (unlike a Sportster engine) due to spring strength.
http://www.secondchancegarage.com/ar...pcv/photo1.jpg

simple, think about it.
we still do the same today
in the industrial world, we set the crankcase pressure to what is needed, sometime even a positive pressure.
it is a simple thing to control. the old open down draft tube would allow water and all kind of shinola into the c/c but with a close system and positive pressure, be hard for stuff to get sucked in even in the seals.

every one is thinking VALVE, not PCV, hugh diff. we control c/c pressure without a so called pcv valve.
for an example, look at the waukesha 7042 engine.

if i have an engine operating in a hostile environment, i do not want that being sucked into the c/c so pressurize the c/c.
the newer cars do not use one either, they use a controlled bleed.