is it the one with a push button on top of the filter housing to prime it? if so touching that primer at all usually causes them to draw air in as they are shite by design lol and when not touched for years the seal becomes brittle so when you do finally pump it they leak air
fit a new one (non genuine ones are shite so get one from ford)
i when changing filters on these use a vacuum pump to suck fuel through the filter, you could also use a hand primer pump between the fuel pump and filter, they are available from most motor factors for around a 5er,
you can either use it to suck fuel through then disconnect it and reconect the fuel pipe to the pump or just leave it on permanently