HOW DO I FIX MY COMBI BOILER?

I have a VAILLANT 242 eh combi boiler installed, and is approx 12 years old. Water and heating working But pressure keeps dropping I fill the loop on a daily basis sometimes more regular I have re filled the expansion vessel to the VAILLANT specs, I have checked for leaks and can't find any. I have spoken to a plumber about It and all he could come up with was i need a new expansion vessel, he would not advise me on how to do the job, so i would appreciate any advice you can give me.