1990 ram 350 having major trouble with air in fuel need help badly

alright guys, here goes as to what me and my 31 yearold son who is a fulltime
mechanic have done to this beast. when we bought this truck 4 months ago we had to install a new head, it ran alright up until 3 weeks ago when it started loading up with air, since this started happening we have installed a new lift pump, replaced all rubber lines and the metal lines between the lift pump and injection pump, had the injection pump rebuilt by a professional shop that everyone local uses. we dropped fuel tank checked out o.k. have been trying to run truck out of a 5 gallon bucket, suction and return and still we are going air locked to the point of the engine dying, by the way when changing head we had injectors rebuilt. i figure we are missing something so silly we will both feel like fools when we find it. my son made the first post and as he said we need help badly.
p.s. i forgot to add that we changed the fuel heater and the drain valve on the bottom of the filter.

 

What are the details? There's a million places it could be getting air in

 

As Soggy said there are many places that air could be getting in. Please provide more details.

First thing is always the fuel lines. Both rubber and steel could be leaking. Usually on steel lines you see the leak. But rubber line leaks can act like check valves; let air in but not fuel out.

Welcome to this forum! Once you figure things out a little fill out your signature for us.

Good Day,
JP.

 

A good place to start looking would be where the steel lines run along the frame. They like to rust out there. You won't see a drip because the holes aren't big enough to drip, but the lines will be damp.

 

Did you replace the O-ring on the fuel heater when you replaced it?