"Gustep and all of the other "Vacuum/Blocked vent" theorists, there is a "low fuel pressure" warning that would no doubt have been triggered (and recorded) should this have occurred."
Note however...
The trigger point for fuel pump low pressure switches can be as low as 4psi... The output of the boost pumps is, nominally, 12psi... so the pumps can, in theory operate down to almost 1/3rd the normal pressure before a warning is triggered.
Also, flow rate can vary depending on the type of 777 boost pump ...even though the output psi is the same. One 777 airline AMM shows the data for 2 pumps. Flow rate for one is 16,000kg/hour. Another is 19,320 kg/hr.
I'd be interested to know what sort of pressures are required to open the check valves on the boost pumps.
Rgds.
NSEU