A couple things come to mind.

#1 what's the condition of the eccentric on the nose of the camshaft? If it's loose or wobbly, I bet that could make them die.

#2 is the vent hole on the bottom plugged? In the FSM it says the pump arm should lock if you block that hole, so I'm guessing if it's blocked it will rupture some internal gaskets.

#3 check the return line for blockage. I'd say get rid of the FPR, but you already did. Again the FSM says blockage in any of the lines should lock the pump arm.


Maybe?

My Truck: 1987 XtraCab DLX 22R 4WD 5 Speed Manual
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"Speed has never killed anyone. Suddenly becoming stationary, that's what gets you." -Jeremy Clarkson