The other day I had an Opel Zafira which gave lean codes and the engine light comes on. I determine that when I unscrew the oil filler cap there is a lot of engine suction. This indicates that the PVC valve which is part of the valve cover is faulty. You have to buy the whole cover if you want to replace it. This cover is quite expensive and the customer asks me to make another plan.
You will see the photos below of how I have fit a pipe from the valve cover breather pipe to a 3 piece metal junction where I have fitted a Toyota PVC valve in series to the intake and the other pipe to the air filter plastic box.
I have drilled a hole into the bottom of the air filter box at the bottom where I have fit that pipe into for the engine to breathe.