worcester 350 combi has'nt been used for 2 years as house been empty, actually got boiler working but after around 1 minute you get a rush of air, pressure gauge rises then boiler shuts down consequently resetting over heat stat on top. tried this on numerous occasions same result cant seem to get air out of boiler as air vent does'nt look to healthy tried bleeding pumps both pumps have water to them, is this the fault any suggestions