Leyland Daf 45/150 fuel problem?

Hi Everyone, I’m new to this group and forum, reason for my joining is that, I’d be very great full of some help and support for a lorry that I’m working on for a friend. It’s a Daf Leyland 45/150 Cummins 6bt (I believe it is) it’s a 6 cylinder Cummins, I know that for 100% but I am sure it’s a 6bt when googling it. Basically you start the lorry up and the idle is rough, then put your foot on the accelerator and it picks up the revs slightly and foot to the floor all you get is a fast idle. When you take your foot off the accelerator the engine stalls and you have to prime it on the lift pump in order to get it started again. Between me and a friend we got her started and I loosened the bleed nut on the side of the fuel filter housing, some air bubbles emerged and the revs started picking up again but then the revs went down and it died. I have checked up and down on this lorry for cracked hoses, etc, replaced the filter and the seals. I also have some new fuel lines coming in the post. Currently there are nylon fuel lines on the lorry and I am replacing them like for like. I mean can nylon lines go old and horrible inside? But after the fuel lines have been replaced if it still carries on what’s next? The whole system is old school. You have a tank and fuels lines, lift pump, injector pump. It’s a simple old set up. I don’t think there is a fuel pump in the tank??? 🤔. However we got the lorry running really nice one day but there was a leak on the injector pump and when the lorry was running it was spraying out of a nozzle on the injector pump. Tightened it up but that leak almost seemed like an escape path for the air? But now I’ve tightened it up and we’re back to square one? Any help please would be really great. I like and enjoy playing with engines but I’m no pro, I am still learning and would like to keep learning. Thank you all for your time and sorry for the long post. Thank you x