I agree with John S that the problem was likely due to a defective drain back valve or something preventing the drain back valve from sealing completely.

I assume that the plugged bit that you are refer is the flame trap on the side of the engine? If so, as long as you have the hose from the oil filler cap to the intake manifold, you won't be pressurizing the crankcase. It will run at a vacuum at idle; but, because the flame trap was plugged, you won't have any airflow which means that you are likely accumulating moisture and crud in the oil - good reason for that oil changed.

In 1971 I think the 1800 came with both D jet and carb options? If you have D jet, the hose from the oil filler cap goes to a fitting on the top of the intake manifold. That fitting has a flow restricting orifice in it, no PCV valve. I can't help you with the carb arrangement.

As a heads up, if you have Djet and if the flame trap was well and truly plugged, cleaning the flame trap will allow air to flow through the crankcase and into the intake manifold. This increased airflow is not a problem; however, it will likely cause the engine to idle a little faster than it did when the flame trap was plugged. Not a problem, just means you have to drop the idle speed a little bit.