Grant Boiler problem

[Noilly]

Following failure of my 4 years old heating system fitted with a Grant Combi 90 outdoor boiler the faults were traced to a failed expansion vessel and a leaking auto air vent valve. ( both have been renewed)I now have a problem with the central heating side. As soon as heat is called for and the pump operates the 1bar pressure gauge falls to zero till the pump switches off and then slowly rises back to just above one bar.
(the pressure was set with everything cold and the pressure vessel checked and set prior to fitting) I have asked several plumbers/ heating engineers what is wrong but no one seems to know the answer.
Can someone out there come up with the answer as it won't be long till winter and I don't want a failed heating system then.

 

[Boilerman2]

Are you sure there is no air trapped in the boiler or heating system somewhere this is indeed a strange fault, have a look at the AAV on the boiler, the small black plastic scew in the side on the AAV should be loose and not screwed in tight, try bleeding all the radiators to ensure no air is trapped anywhere. set the water pressure to 1.50 Bar (if the boiler is hot which it should be) at this setting all should be OK

Grant Service Agent