when you say release the air from the valve do you mean open the pressure relief valve and let water out as you should not be releasing air from vessel?????????????
if you mean water pressure keeps increasing even when boiler turned off then filling loop passing or exchanger has failed. disconnect filling loop as it should be...(temporary filling loop) to see if its passing even though it appears to be turned off. if its ok then you may have a slightly dearer problem of exchanger.
or if you have been releasing air from tyre type valve then you'll need to find some more and put it back :-)
iain
June 2008