I'm not sure what getting a 'prime' feels like on these cars. The primer pump doesn't really change in feel once its primed. Instead you usually listen for a change in sound. A squeaky/squishy noise usually indicates that you're there or close anyway.

Some of these cars will run with the pressure relief valve on the injection pump wide open or close to it. Close off the return line and see if pumping resistance forms up is one possible test.

If both return check valves in the lift pump where not sealing in a reasonable fashion You would see surging back and forth I suspect in the small primary filter when pumping.

Of course you could have a defective primer pump but if so my guess is it probably would leak. That depends on it's internal design I suppose.

If the injection pumps relief valve is in decent condition you should feel the pumping pressure increase until the point it opens. I have not had to do it with the newer style primer pump but logically somewhat the same rule should apply in my opinion.