89 f150 5.0 With my engine running my fuel pressure is around 35 psi.With my engine off key on, it's only about 10 psi which makes it difficult to crank and impossible with the engine warm without priming throttle body. 3 new fuel pumps and fuel pressure regulator. What could cause this?

 

Fuel distribution reservoir possibly?

 

Three things
The pressure regulator might be bleeding pressure off
The fuel injectors might be bleeding pressure off
Any of the 3 one way valves nay be bleeding the pressure off
Check the pressure at the crossover valve
Go through some of the "electric fuel delivery" diagnosis in the H manual

 

 

There are roughly 40 pages of that fuel diagnosis in the 1989 PCED (H manual)
AllData at your library is where to get the stuff without buying a PCED

 

What's the voltage at the battery posts when the truck is running?
What's the voltage at the battery posts with the key on engine off?
What style of battery terminals do you have? If it's the clamp style in the picture, they cause issues with loose connections and corrosion.

The two grounds that the green arrow is pointing to, need to be attached to the battery negative terminal. They are pins 40 and 60 at the ECM and the ECM relay coil ground.
What does the voltage at the battery drop to while cranking the truck (while the starter is turning the engine). If that voltage drops below 10 volts, your battery is bad.
Disassemble and clean the connections for the main ground cable that runs from the battery negative terminal to the front of the engine block and the frame. That cable can corrode internally and cause problems. It may need replacement.

 

Reading in the forums that I could possibly have a stuck injector bleeding pressure off . It said that if I pinched off the return fuel line with my vice grips and then keyed on and read the pressure. I could tell if it was an injector leaking, or the problem was at the tank. If it held pressure with the return fuel line pinched off, then the problem was not an injector. Well, I put the vice grips on the return fuel line and had to key on several times before I would build any pressure at all and it would finally work its way up to ten psi. One more try, and it wouldn't build any pressure at all.Which baffled me. For some stupid reason, I decided to pinch off my feed line that goes to the fuel pressure regulator and try it. Key on, and the pressure suddenly went over 100psi. Scared the c*** out of me and I keyed off. Unpinched the line, and then after the high pressure everything was working perfectly. Key on and fuel pressure jumped to 35psi. I can only come to the conclusion that something was stuck in a fuel injector in the open position or an o-ring on an injector but I've never smelt gas. But whatever the thing is working now, after a year of hassling and changing parts.

 

It could also be the one way valve in the pump bleeding the pressure off
3 fuel pumps later, on my last 04 F150, I put one of these (one way valves) on the output line of the fuel pump at the tank

These were for a recall back in the day, but they work for all of the newer ones too

 

If yours is pre-1990, you should be looking at the fuel pressure regulator (on the fuel rail) or the in-tank fuel pumps for the pressure loss, possibly the above-mentioned crossover valve.
They gave us these kits to do fuel injection cleaning
In the kits were fuel block offs to prevent the Ford injector cleaner from getting back to the gas tank
You could use the store bought blockoffs or make one to test your fuel system

The one on the right is one of the block offs.

 

I’m more interested in how you pinched the supply line and built 100psi. Where did you pinch and where was your gauge?!