Central Heating pressure loss problem - HELP

[Peter Hawkins]

Boiler Vaillent VC183EH
pressurised system running about 9 radiators and an underfloor system (separate pump) for one room.
Problem with pressure loss needing frequent topping up. No leaks found although there are some runs under the suspended floor that I can't get to.
No air in rads.
First noticed that the auto bleed valve at the top rear of the boiler ( Brass float valve) was dripping. Replaced it.
Then noticed that the pressure release valve that vents to outside was dripping. Replaced it.
Air got into the UFH loops. Vented it out.
Now it runs fine, and loses no pressure until the boiler goes off. Next morning pressure has gone. I open the filling loop and get the pressure back up to 2 bar and everything is ok until it goes off at night.
If I stay up after it has gone off, and the pressure has gone, and top it up before I go to bed, it is still OK in the morning.
Predictable problem so I can keep the heating running, but need to fix it properly. I have got some leak sealer to put in but don't want to until I am sure that there are no other problems.
IN SUMMARY - IT ONLY LOSES PRESSURE WHEN IT IS COLD
Help!!!

 

[daden]

test pressure vessal is 1 bar if not open prv and pump up 2 1 bar

 

[Peter Hawkins]

The pressure vessel is the flat red tank at the back of the boiler?
How do I test it?
The boiler has a system pressue gauge. This is what goes down to zero. I add water via the filling loop to increase the gauge pressure to around 2 bar to get the system going again.
Are you suggesting this, or some other way of pressurizing the vessel