Pool pumps are just simple centrigugal pumps. They are designed more as simple water movers than pumps, with very little head and pressure. Any leaks at all on the intake side will cause problems. If your water just "goes away" with no visible sign of leakage, check for an air leak. Could be as simple as a hose or clamp, or a small crack in a filter housing, pump housing, or solid line somewhere. Something that I have used to repair cracks that works very well is a 2 part epoxy made by permatex. It comes in a setup that looks like 2 hypodermic syringes attached side by side. You press the syringe, and it dispenses the exaxt amount of epoxy and hardener together. Rough up the surface with 120 grit sand paper, wipe with mineral spirits, mix up your epoxy and apply to the crack on the moderate but heavy side. Allow it to dry for 10-14 hours, and have at it! I fixed my own sand filter housing, and my pump head housing both back in 1996, and they are still solid after 10 years.