There are many sources of high vacuum leaks. A hair or piece of lint lying across an o-ring after recent maintenance can cause a relatively large leak, as can an o-ring not properly seated, dirty, dried or decayed. Be aware that contamination within the instrument (such as a fingerprint on a source element, or residual solvent from a source cleaning) may cause a high pressure, but no leak will be found. Also, after a Cold Start, residual air in the instrument may take several hours to remove by the pumps, so do not confuse 'normal' residual gas with a leak.
Excessive carrier flow through the column may also cause high pressure in the mass spectrometer. Carrier flows of 1-2 mL are typical for instruments configured for capillary direct (no separator) interfacing of the GC and MS.
There are two broad classes of high vacuum leaks: those that preclude turning the MS ON, and those that are small enough to allow the MS to be turned ON.