Barit1
The low pressure pumps are something very much like this....
http://www.eatonaerospaceltd.com/fuel/Resources/031_2_Type9602boostpump777_300.pdf
I beleive there are non return valves fitted in both the inlets and outlets and it is one or more of these that I am suggesting as being complicit in my post above.
There are two such pumps on either side of the aircraft and a smaller very similar one (on the left side I think) which is specifically for the APU.
JG