Worcester 25/32 oil boiler problem

[1UKDUDE]

I have a problem with my 25/32 boiler pressure
If i fill it up to 1.5 bar when i run the hot tap the pressure drops under 1 bar !!
I have checked the expansion tank and its good and i have re pressurised it
as soon as the boiler cools down the pressure increases

Any ideas

 

[Exedon]

When you re pressurised the expansion vessel did you de pressurise the system?

 

[1UKDUDE]

Thank you for your reply Exedon
I emptied the system then I removed the expansion vessel took it outside and pressurised it to .5 bar then refitted it and filled the sytem back up to 1 bar