Isuzu Trooper Owners Club UK™

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Hi all, I'm in NZ, don't know a huge amount about diesels, could really do with some advice so trying to reach out to my homeland experts.
My Son (14) has been working multiple part time jobs and managed to buy a 1996 3.1TD Bighorn 4jg2. His dream is to restore it, raise it, snorkel & muddies (you know!) Get it back on the road with new WOF (MOT) for when he gets his licence.
So first job was to strip it back underneath to work on rusty chassis. Drained & removed fuel tank, cleaned ends of steel fuel lines & taped over to protect. Removed & cleaned nylon pipes from tank, treated & painted outside of tank. Chassis all treated, fixed up & painted, so time to reassemble nylon pipes taking care of right way around & re-fit fuel tank. We hadn't touched the fuel filter or lines in the engine bay, so from what I read, we didn't need to bleed anything. Anyway, cranked it over and it started after a few turns, let it idle for some time before applying throttle and it seemed to be running perfectly. Job done right?
Next job to cure some coolant leaks - new thermostat gasket and a hose. Again, started perfectly. Ran for some time to warm up & check we'd fixed the leaks - All good.
It has been started & run for a short time (Less than 5 mins.) three times since then.
Then, a few days ago it wouldn't start. Cranked, fired up but died immediately.
Then the weird thing: I tried starting with foot on throttle, it started, went straight to red-line revs. like infamous "Diesel Runaway" but thankfully shut down when ignition turned off. This happened three times before we decided to stop and have a think.
Decided it may be fuel filter so removed outgoing fuel line and hand-pumped purge valve - Diesel came out of outlet. Didn't go any further trying to bleed anywhere.
The filter looked old so we thought we'd get a new one anyway. Installed new filter and tried to hand pump again but no diesel came through. Undid incoming line, no fuel in there, so removed filter & filled it with fuel assuming syphon effect would draw fuel. Still nothing. Maybe the fuel line blocked? So disconnected the line by the tank and blew compressed air through from the filter end - no blockage.
Put the lines back together and still got no compression from the hand pump. Took out the 12mm bleed bolt in the centre of the four injector pipes (Read this online) and cranked it over a few times. Diesel was ejected from the bolt hole but suspect this was just what was in the filter as the incoming to the filter was still dry after.
So really confused: Should the injector pump have enough suction to pull diesel from tank to filter? And what's with the red-lining?
Can't afford a mobile diesel mechanic, so any input would be awesome. Thanks.

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