combi problem pressure drop

[Dewalt2003]

Hello have a valiant ecotec 35 Combi, started to lose slight pressure on a daily basis approx 0.2 bar. had a look around and found some crustation around expansion vessel, only 2 yrs old so phoned valiant and engineer said it was no good and this was probably the problem.
since it was replaced under warranty I have been losing more pressure approx 0.75 bar per day.i shut of flow and return valves under boiler and used for water only over a couple of days.
No pressure drop. when I re opened flow and return, big gush of water from flow and pressure dropped from 1.6 to 0.8. topped up to 1.5 and within 24hrs boiler had locked out at 0.6.
assume I have leak under ground floor some where, was going to get a leak specialist in to find as I have 3 rads in a screed and 5 others all under amtico flooring.
but since topping up this morning back to 1.5, heating been on and water used and pressure has stayed the same, has anyone got any ideas please ?. normally this would have dropped by at least 0.6. this been ongoing for the last month. checked prv and condense permanently dripping but both ok. always had a combi since boiler rads and pipework installed.

 

[ianmcd]

checked prv and condense permanently dripping but both ok

what do you mean by that ? do you mean the PRV is constantly dripping ?

 

[Dewalt2003]

No , prv ok bag over it ,no water. condense not permantley dripping, heat exchanger ok.
I'm now topping up at 7 am in the morning, holding pressure all day, checking at 10 pm and still 1.5 bar.
but when I wake up 6am. All pressure gone form boiler, showing 0.5 at lock out.
when I top up the pressure shoots up to 1.8-2 bar within a second.