Antony, how long exactly is an "away"Yeah what he said ^
You might get a build up of calcium on the impellors of teh pumps. As Wee Man says, every six months or so strip the pumps and soak the impellors in warm water with vinegar for about an away and then scrub all the muck off.
Just a word of caution.
After doing major service / cleaning work to your skimmer it will perform better. Don't put it back together and rush out somewhere. You will come home to a flooded cup and worse if you have an auto top, it will flood the tank.
You will find that you will need to re adjust your skimmer, so be arounnd for a couple of hours
No, that's the airline for your venturi, your pump sucks in air through this to create the bubbles in the skimmer, that is why it is a good idea to clean them occasionally as they can get salt creep and little insects in them blocking them up and reducing the efficiency of your skimmer.