Hi, does anyone have any idea of common faults causing constant air bleed back to the fuel tank? The tank has been cleaned of sludge, filter replaced, gauze cleaned, all joints checked etc. When on prime or after driving , air still bleeds back to the tank when seen through a clear pipe (image below). New air bleed valve and non return valve fitted. Defender doesn't get up hills, runs out of diesel, turn around and go downhill and it runs again. Fuel pump has now been changed, still the same with air in system. Is it common for pipes to split with No diesel on the floor? Grateful for any pointers.

Is that a TD5 I see in the picture?

How are things at the fuel pressure regulator? Any sign of any leaks? Does it maintain fuel at the correct pressure? I suppose another place where fuel could get out and air in is the O ring seals around the injectors. In which case the oil level in the sump would tend to rise. There's pressure in the system when the pump is running, so it's unlikely to suck air in when it is running. And if there's air getting in when it's off, that would tend to leak fuel out when it's on.

Another problem I've had is the seals on the fuel filter. The centre one can get rucked up, presumably when the filter is screwed on, and this can cause intermittent cutting out and poor running. So even if the filter looks snugly fitted and isn't leaking, fuel may be passing from one chamber to the other.

Thanks for the reply. Yes, it is a TD5. The engine runs really smooth and pulls really well, until under load or on a hill, then it starves itself of fuel. I put a clear line on the bleed line to watch it when running, I can run it for 1/2 an hour and still pushes air out of filter assembly. I am going to look at the higher level fuel regulator and pipes as you said. Oil in sump is clear of diesel and zero leaks of diesel.
It starts first turn of the key after warmed up, but if left for 2 days or more, needs priming. New nrv has also been fitted. I'll go over it again tomorrow, thank you.