No it would be the water pressure reducing a little, as the it loose heat. If constantly drops till it reaches 0 then you have leak, other wise it just a pressure fluctuation.
As the pump and heating water increase the pressure.
Then I am afraid I have t tell you to ring some one, to fix the boiler. As you have compromised the combustion chamber, and if you don't put things back correctly, it may cost you more than a call out fee.