How to pressurise fuel line to find air leak?

[Graham_Wright]

I have electric pumps in my port and starboard tanks. They are built into the bilge keels so low down. I have air ingress and have tried all ways to find it. I have come to the conclusion that air is sucked in by the engine and heater pumps. It is possible to imagine a mechanism that would allow a suction ingress but not be detectable by pressurising. No amount of pressurisation has revealed a leak.
I have now replaced nearly all copper pipes with appropriate hoses to eliminate as many leak sources as possible.

 

[Moodysailor]

I have electric pumps in my port and starboard tanks. They are built into the bilge keels so low down. I have air ingress and have tried all ways to find it. I have come to the conclusion that air is sucked in by the engine and heater pumps. It is possible to imagine a mechanism that would allow a suction ingress but not be detectable by pressurising. No amount of pressurisation has revealed a leak.
I have now replaced nearly all copper pipes with appropriate hoses to eliminate as many leak sources as possible.

Hi, this is possible, but is rare. I have come across this once before where we found a leak that was only apparent under vacuum. Hard as it maybe to believe, the theory was that the difference in molecule size was the factor - air is just less dense than liquid.

It could also be that the leak is via something that closes under pressure, but is open under vacuum - i've seen a fuel lift pump diaphragm do that in the past too.

These types of fault are notoriously difficult to find. At the end of the day, if they were simple to eradicate, I doubt there would be much need for a forum post

 

[Keith-i]

Based on what I've gleaned from the bits of advice and experience above I shall disonnect the outlet from the engine filter/primer pump and fix a priming bulb onto the filter. I can then either turn off the fuel valve or block the end of the tank pickup and pressurise the majority of the sytem. I shall return with my revelations in due course!