A6 Allroad - anyone know what this fuel part is?

[jpp]

All I know is there's a weep of diesel that's appeared over the weekend from the bottom end of it and there's now a small pool of diesel on the bracket under it. I suspect it won't be doing anything much good around it and it stinks.

Inch long cylinder in the centre of the picture, which is roughly just below the engine's centre line and above the fuel filter.

[centralscot]

Non return valve?

[jpp]

Seeps when the engine is switched off. Seems like you can't just buy that part, typically.