Barn of Parts Frozen Roll Over Valve Solution

[Confused]

Ring seal is not perfect. There will always be a small amount of blow-by, even in the best engines. (This is what a cylinder leak-down test shows - how well the rings are sealed). Also, as the engine oil is heated, it expands and generates some oil mist/fumes/vapor. There is a small amount of pressure in crankcase/engine. That's the reason for the vent line that goes from valve cover to airbox.

Thanks, I'm still curious what force is on the crankcase in our engines. Now that I have an easy way to tap into it, next time I have everything apart I'll put a gauge on it.