Air leaking into fuel diesel line?

Hi has anyone out there had problems with air getting into their Peugeot Partners 1.9 diesels fuel line.So far we have changed the pump/filter housing and had the whole system pressure tested and put a non return valve on the supply fuel line to no avail.The car will not start when been left for a few hours but once started will start ok again until the next morning.Any ideas would be most welcome.