Non return valves are not a cure for air leaks. Where old fuel lines are replaced and yet air leaks or back flow into the tank still occurs when the vehicle is left standing for a time, then back flow valves can help out in a short term fix in some cases. I have not had any problems leaving a backflow valve in the fuel line permanently.
The valves can be a bit hard to source but some diesel specialists locally can get them in.
I have posted a photo of two sizes in case that is of some help.
http://www.farmphoto.com/thread.aspx?mid=881543&cb=1289023523901#881543
Road vehicles often have a spin on filter in the line before the lift pump. If the seal on these is faulty then air can get in to allow back flow in some designs.
The valves make the system secure regarding back flow but still any such leak does need attention or air may be sucked into the pump.
The valves can be a bit hard to source but some diesel specialists locally can get them in.
I have posted a photo of two sizes in case that is of some help.
http://www.farmphoto.com/thread.aspx?mid=881543&cb=1289023523901#881543
Road vehicles often have a spin on filter in the line before the lift pump. If the seal on these is faulty then air can get in to allow back flow in some designs.
The valves make the system secure regarding back flow but still any such leak does need attention or air may be sucked into the pump.