Baxi Combi pressure problem

[jacksparrow]

Bled my rads, dropped pressure to 0, so increased it (to 2!!!). Since that there was a drip under pump? Service engineer came out recharged expansion vessel, replaced o ring and drip stopped. However pressure drops every night from 0.6 to 0 (but still have hot water and rads fine). Any idea, boilerman says I have a leak, expansion vessel is holding its charge but is this a coincidence? He suggested locating leak or putting in leak sealant. Help?

 

[ChrisR]

WHat boiler?

Checked the pressure relief pipe through the wall?

 

[jacksparrow]

Is that pipe black cone shaped

 

[jacksparrow]

Boiler is Baxi Combi 80