There is also a gause filter in the lift pump which often gets neglected. Remove the dome on top, one centre screw. Don't remove rhe seal in the dome unless you have a new one as they expand with time and you'll have a job to refit it. Pick out the gause filter and clean it by dabbling it with a wee paint brush with white spirit, petrol, diesel ect. Beneath where the filter sits are the sediment chambers, these need cleaning out, ideally blowing with compressed air, or blowing through a bit of tubing by mouth. reassemble and bleed at the filter on the engine.