old saunier boilers all leak from many places mainly the divertor near the pressure gauge.
it is caused byexpansion tank losing back pressure & damaging the divertor.
change divertor & when boiler is empty pump air into expansion vessel (red tank on the back) with foot pump as it is a tyre type valve. put it up to 1.3ish bar.then boiler will work & not cause leaks randomley due to expansion pressure pushing on joints or 3 bar valve but int expansion vesell as designed.
daniel servante, December 2008