My test for a clogged box is to remove the oil-filler cap, run a hose down to the separator (still installed on the block), gently blow air through it. If it's free, it passes; if there's noise (from chunks moving around), or resistance, I pull it and clean/ replace.

Does this seem a reasonable test?

I used to clean/replace all of them but I found that if air passed through easily, I didn't get much out of them: fill with gasoline, soak several days, tap gently, shake.

This came up recently when I worked on a car with a rear-main and a front cam-seal leak. Box was plugged solid - no air flow. I replaced the cam seal as it's easy; we'll see on the rear-main.
--
240 drivers / parts cars - JH, Ohio