I was going to say the same thing and add one further. There should be a switch in front of the pump for dry and wet pumping. Make sure that switch is ALL the way over to Wet Pump. Mine frequently feels like it is, but I have to sometimes flip it to Dry, then back to Wet to make sure it went all the way over before I pump.
What are the conditions? Does it pump and not produce anything? Or can you not move the plunger at all? I'd check the seacock and the switch before rebuilding it. Though, keep the rebuild kit, you will need it eventually.