Hello, I am having a hard time getting my oil pump to prime and stay primed. The pump is not badly worn. I've heard of packing the pump with light grease to prime. What could be the cause of it not staying primed? Thanks to you all. Dex

There are two reasons that it may not stay primed.

The first is low oil in the crankcase.

The second reason is that there may be some debris caught in the oil distribution valve assembly. This unit is located in the block.

Hey oneseventyone Sounds like you may have a air leek in your oiling system, or a plugged inlet/outlet.
Check all your oil connections,right up to your oil preasure guage and see if this fixes the problem, if this does not fix your oil problem check oil pick up in the oil pan, it may be clogged.
Also check the oil pump blade spring's ,inside the oil pump for broken or week springs and reseal oil pump gasket if you open the pump up.
Hope this helps your oiling problem.

Keeping vintage Chevy alive.

Another possibility is that the rotor that carries the spring loaded vanes has excessive wear/clearance in the body of the pump itself.
The rotor has 2 diameters where it runs in the cast iron pump housing,and you'll usually find that there are 2 grooves worn in the pump body itself.This may contribute to loss of priming and/or oil "pressure".
It's an easy fix if you have a lathe,or access to one,just set the housing up,bore it out(usually a 1/16" cut is plenty),then make either a phosphor bronze or cast iron bush to suit.With a couple of thou clearance this should solve the problem.
But as previously posted,check the spring that fits in the vanes,they can fail.

Thanks to all of you. The spring for the vanes Is okay because I replaced it with an extra I had from another engine. I rebuilt a pump like this many years ago that was worn way more than this one. I've got to have an air leak because this is a fresh rebuild any everything Is clean. I've tried the pump out side the motor with a drill and it worked.