Agilent has some pretty good online app notes, as well as phone troubleshooting. I'd try that first.
I'm used to the older agilent instruments. Please confirm that the leak is showing up as high pressure in your vacuum line. If so, the most common source of air leaks with the older GC/MS is the ferrule around the columns. If they are just a little off, your vacuum goes to heck.
If it is water, then your best place to look is at the drying media in your helium line. Of course, your vacuum line or vacuum pump can also cause problems, especially if the instrument hasn't been used for a while.