Vehicle will crank but will not start when left on a steep level

Hot start is perfect. Only cold start issue. If it was left on flat or down steep (engine lower), it's works perfect, no issue.

It will crank but won't start when left on an up steep level (engine higher) for a few hours or overnight.I think it's a fuel related issue because one way to make it start that always work is by pumping the manual fuel pump to refill the fuel filter tank (seems empty by sound it makes) and fuel pressure in the tiny holes. It seems like the fuel is returning back to the tank.

It's a 1.9D M59 berlingo model.

I checked for a possible leak in the fuel hoes, no visible leak. I changed the manual fuel pump, no effect. Is there like a non returning valve situated somewhere which might be faulty in stopping the fuel from flowing back.

Please advise me what can cause this.