How can i resolve the loss of cold fill pressure?

go to merchants get FERNOX LS1 (or similar)
THIS IS A LEAK SEALING CHEMICAL.

getting this in the system is not easy without a pump -but you may disconnect a radiator to put it in. It won't be easy.

the fact you can not see a leak does not tell you everthing. It is possible and indeed probable that you could be pulling air into the system -ye sounds silly

I assure you I have 30 years experience in the field and 15 in the lab -- it happens - ait entrainment - more than likely near the boiler itself.
LS 1 does usually work - follow instructions carefully

dave broadbent
June 2007