Same thing happens every time with mine. I have a Chinese one from before 1990 and a US made one bought about 2009. The same issue can happen to a Fuel Injection Pump that has sat on the shelf for years or even one on a Car that has sat for years.

Ones still on the car with all of the connections intact seem to do much better because there is no place for the fuel to evaporate because it is sealed off. However, sometimes rust forms inside the Fuel Injection pump due to the water content in the fuel.

Note WD-40 or some other penetrating oil after the delivery valve is out works to free them or at least help. If not an aluminum or brass punch gently tapped on the top of the plunger usually frees it.

__________________
84 300D, 82 Volvo 244Gl Diesel