Real ca18det catch can/ popping dipstick

[TheMAN]

excessive pressure (positive or negative) will just pull the seals inwards or push them outwards
physics 101

[mdb4879]

Right, but there's a backing behind every seal that will prevent them from going in any further, and the crankcase is under vacuum when cruising. At idle I would figure a rough estimate to manifold vacuum is 22in/hg, which is about 4psia, or about -11psi (vacuum). You can't go any lower than 0psia or -14.7psi (dependent on sea level and other factors), and since the crankcase and intake manifold are connected then as long as the pressure in the crankcase is greater than or equal to that of the manifold the PCV valve will connect the two so when cruising or at idle if there is a vacuum in the manifold there will be vacuum in the crankcase. And this is how cars are set up from the factory.

[TheMAN]

the pressure differential isn't great from the factory because you have a breather hose hooked up (fresh air always going into the engine), and the PCV valve regulates the amount of vacuum

the backing can only do so much

[s13drifter88]

Google an FSM, plumb like stock, enjoy