For the system to work properly,... it must be perfectly dry, and free from grease, WD40, vaseline, lard, monster munch, monkey spunk... whatever you use. If you're cleaning the cap out.. don't use WD40,... use brake cleaning spray. it degreases and then evaporates without leaving any nasty stuff behind
I shall describe the best method I've used so far:
Get some 800grit wet 'n' dry and clean the bottom of the dizzy cap, and the surface it mates with on the dizzy body. Only apply light pressure, you're just trying to clean the surface (if it's a new cap... just do the dizzy body as the cap should be flat already)
Apply a light bead of silicone around the mating surface on the dizzy and let it cure.
Now when you come to fasten the cap down,.. the resistance from the silicone creates a perfect seal.
It's a fiddly method, but i took the time to do it and it worked. WD40 is fine as a water displacement fluid,... but it can cause more harm than good if used religiously every slightly damp morning.
failing that... reach for the pair of marigolds and 'laccy bands