Sounds 100% like a
fuel line blockage, probably rust and crap from in the tank, has been sucked into the outlet tap when the tank was drained.
Unscrew the tap and blow it out with compressed air.
and check inside the tank (sorry you will have to drain it again)
While the pipe is disconnected from the tank, also remove the pipe/hose off the pump inlet and blow back with compressed air (from the pump end)
Also check the Pump for blockages....... note you cannot back blow the pump, you will have to blow out in the direction of normal flow, so disconnect pipes off both inlet and outlet.
When is the last time the filters were replaced?
could be blocked filters from all the crud in the bottom of the tank getting flushed through.
The blockage will be between the tank and the first filter, so will not be too hard to find