The cap should be snug on the reservoir. The vent is that small point in the center of the cap.... has a spring on the underside that vents pressure if it happens to build up in the reservoir.
I would clean up the pump with some degreaser or whatever your favorite method is, and next time it leaks look carefully to see where the leak is coming from. Probably a seal is starting to deteriorate?
Try some power stearing stop leak. This swells up the rubber seals so they, well, seal better
This worked for me. I hear adding a bit of brake fluid in to the ps reservoir does the same thing, but the stop leak fluid is only a couple of bucks at the local auto store.