Way back when I was having the first round of issues with my DZ170cdi, I stretched the springs in the pump so the valves would seat with more pressure.

While it worked fine in my hand with a large piston stroke, it turned out the vacuum required to open the valve on the suction side couldn't be achieved if/when a bubble got into the pump by the much shorter piston stroke that the intake pushrod imparts . Imagine trying to draw water up into a syringe that has a bubble in it, the piston displacement needed to overcome the volume change in the air bubble just wan't enough.

Just another one of those interesting nuggets of discovery..