Airboxes? Checkvalves? Do you have a D.D. engine.
If they are what I think you are referring to they are oil drains back to the sump and should open upon engine shutdown.
Often they can be cleaned with diesel or some other solvent. It is usually goo from old oil and some dirt that sticks them up.
If they still won't work then you may need to replace them.
And no , I'm not a D.D. guy, just have run across mention of this problem often enough that I remembered.
Also tell what the engine is, don't make us guess.