1998 Ford F450 needs a squirt of fluid to start

My truck wont start without starting fluid but runs fine with a squirt. Already checked the fuel pressure in start position, its also firing in start. Seems to be injectors not injecting in start mode. But why?

I suspect your fuel pump is getting weak

It’s clearly strong enough to keep the engine running, but not strong enough to actually start the engine without help

7.5 liter V8 gasoline?

What’s the fuel pressure key on, engine off?

Is this the original fuel pump?

Yes it is a gasoline engine. It has a 7.0L 429. It is actually an F700. I believe the fuel pressure was 40PSI on the rail.

Not sure if it is the original fuel pump…

If you turn the key from Off to Run (not all the way to Start) and after a few seconds back Off - and repeat this a couple or few times - the fuel pump should run a couple seconds each time. That may bring enough fuel and fuel pressure to the engine. Now turn the key all the way to Start. Does the engine turn over? Does it start? Does it keep running?