Even though I applied the zip tie, I don't think it is the solution.

When I removed the siphon pump, mine was intact with intact O-ring, zero issues. Everything aligned perfectly from factory.

I still think the O-ring shrinks with time, allowing it to bow under pressure.
When I disassembled the Coupler, the part slides out easily, telling me that O-ring has reduced in size. It should have been tighter when sliding it out.

Anyway, the new O-ring is much better, a tight fit.

The one-way valve is over engineered, if you remove the one-way valve can be removed without issues.

I understand that you did a lot of pioneering work in the fuel siphon pump issue, kudos to you. But I don't think anyone here knows for sure the EXACT cause of failure...

Everything here is conjecture...

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1998 E39 528i 5sp MT
2006 E53 X5 3.0 6sp MT