400 diesel supply pump problems

Currently working on a farmall 400d. Problem is no fuel pressure reading on fuel pressure gauge. Thought maybe a bad gauge tried that, cleaned fuel lines, filters and anything restricting flow. Still no pressure but, the tractor runs perfect on diesel. Snap the throttle and she belches back smoke and runs smooth as silk. I disconnected the discharge side of the fuel supply pump located on the rear of the pump, and with the tractor running on gas I let the line discharge into a bucket. When I tried to block flow it was very easy, just by resting my finger under the line I stopped fuel flow from the supply pump. I feel as thought this is my problem being a weak supply pump but im not 100% positive. I purged air where the discharge side of the supply pump connects and by squeezing the trigger on my air nozzle I got a good reading on the gauge and lots of fuel pressure like it used to be, because this problem occurred recently. It used to read fine, and bleed strong blowing fuel everywhere when I bled the system, but it got weaker and weaker over time. But whats amazing is this problem has not effected the power at all, or It could be just feeding off gravity.

I had a similar problem with one last year. Turned out the tiny hole in the back of the gauge was plugged. Sounds like that's not quite the case since you can register air pressure.

I did re-do the bushing on the lift pump as they were worn. Mine appears to have an earlier style pump with a lift and a scavenge section. If you have that could you be leaking fuel from one side to the other?

I do know that its not a dual pump running 4 gears. This one has just 2 in the back.

I had the same problem with the same symptoms. The supply pump drive pin was sheared.

May be some crap under the return check valve which is set to open at 40 to 45 psi, same as the regulating valve in lift pump. Check valve is under the nut, and possibly snap ring and disc on top end of pump. lift pump relief valve is under the cover on rear of lift pump. Broken spring, stuck valve same problem.