So no compression test yet?
Well how about an idle vacuum test then.
PCVs work the other way. In your case the calibration spring may be stronger than your vacuum is able to overcome. This suggests that your idle vacuum is low, not high. But if it rattles when you shake it then that suggests that the calibration spring is not too strong, again suggesting that the idle vacuum is weak. Cranking up the Idle timing and having the engine run better also suggests the idle vacuum is weak.
This is called chasing your tail.
