First of all, it’s normal for the reading to increase after the system is isolated from the vacuum pump, and a rise in microns doesn’t necessarily mean you have a leak or that the gauge is defective. Assuming you’re using a digital gauge (and you should), it’s highly accurate and extremely sensitive to what’s happening in the system.
If the reading rises rapidly at first but levels off in the 1000-3000 micron range, chances are the rise is related to the out-gassing of your charging hoses. One solution is to use a 4-in-1 core removal tool, which lets you completely isolate the charging hoses by giving you the ability to valve them off and measure the vacuum of the system only.
If the reading rises at a steady rate and doesn ’t level off, then you probably have a leak. You’ll need to find and fix the leak, and repeat the evacuation process to make sure it’s fixed.
Source: The BUZZ by Yellow Jacket