Right. It's a crap shoot. Literally. Every time those 3 way valves port to atmospheric pressure the volumes they're connected to are equalized by what can be a dirty environment. I prefer to avoid sucking crap into the valves because I've had to disassemble and clean a few from cars with missing filters. Course, you can delete the valves altogether as many people have. If your state does enhanced emissions testing you're going to need the EGR one though.