What antifreeze are you using? The pink Hoat or DexCool? It's been my experience that they don't have enough lubrication properties for the old style Magnum pumps. Stick with the old style green Prestone aka Ethylene Glycol.

1970 Plymouth Barracuda 1994 Ram 1500 4x4 5.2L 2004 Ram 1500 4x4 5.7L Quad Cab