206 conventional diesel engine

I have problems with start in the morning, especially when the car has less than 1/2 diesel tank, it is as if air is entered into the circuit, or the diesel would return to the tank .- I tried an anti-return valve placed before the diesel filter, but there were no changes.-That test advises me to do or which can be the inconvenience?

Check the reforador and pipe filter until the arrival of the gas-oil filter, especially the clamps or have no cracks at any point, the injectors must be watertight, put a transparent tube at the pump entrance and see if there are air bubbles ... make sure that it does not take air through the filter joint, then there is the status of the heater and that it is at its extent. a nonsense) ... greetings !!