Ed is right, there is a spring that only allows the vapors to be pulled from the crank case. The spring slams the valve shut to should something present a pressure Greater that crank case pressure.
It is how the valve works.
Test: start engine and allow to idle. PUll PCV valve and note change in engine RPM not to exceed 200 - 500 RPM. Place finger over end of valve and feel valve shut. Vacuum draw on finger should be no greater that 12 - 20 inches of mercury and steady ( this is vacuum seepage ). Release valve and note the valve should sham open and vacuum should be the same as manifold vacuum.
Clean with approved O2 sensor carb cleaner.
Just my $0.02 worth.