SStones Said:

If the boiler is overfilling, you'll want to take the boiler probe out and clean it.  It will be located on top of the boiler, it will be a hexagonal fitting screwed into the boiler with an electrical connection and one wire plugged on to it.  Don't just yank the probe out of the fitting. Take the wire off trying to make sure you don't move the probe mounted through the fitting. Then unscrew the fitting out of the boiler to see the bottom "Inside" end of it.  It will probably be covered with calcium if the boiler was overfilling.  You can simply clean it off with a wire brush, sandpaper or steel-wool. Screw it back into place with a bit of teflon tape or one of the loctite thread sealants, reattach the wire and turn the machine back on.  Once it is hot enough to pressurize the boiler, open your hot-water valve to lower the water level in the boiler.  When the pump comes on, the water is below the probe.  Close the water-valve and when the pump shuts off the boiler is filled to the bottom of the probe, the correct level.
Now...  If the boiler has been over filling, I'm suspecting that your leak is actually comong out through the safety relief.  The bigger thing that looks like your anti-vacuum valve.

To locate a tiny, hissing leak on a boiler. Put a butter knife in your freezer for a half hour. Take the top off of the machine and turn your machine on.  Once the machine is hot put the cold knife tip near the boiler connections and near the valves on top.  The steam will condense visably on the cold knife, you will see it.  If it is the safety valve it might be best to replace it or have a technician replace it.  We're not supposed to suggest you try to clean and set it yourself.
That said, you can set your pressostat yourself and so long as the safety isn't leaking at all at 1.2 Bar, and your boiler pressure never exceeds 1.2 Bar, then Bob's your aunt's fiance.

Posted January 23, 2013 link