why does it still not work?

Having recently fitted a new air pressure switch to a "Vokera Excell 80sp" it still does not work, i can get it to fire up if i disconect one of the tubes and suck on it but as soon as i replace the tube the boiler cuts out, could it be the p.c.b? as the fault seems to indicate that it is the air pressure switch? many thanks m.edwards.