The end of the water pump shaft is probably protruding up above the impeller surface.  A few thou. protrusion keeps the rubber faced sealing washer from sealing up against the face of the aluminum impeller.  If the shaft is protruding the impeller needs replaced or you can grind the end of the water pump shaft so that it is even or a few thou. below the impeller surface. 

The problem described above is responsible for about 95 % of the LTs leaking coolant out of the weep hole on the bottom of the water pump.  The ceramic faced water pump seal seldom fails.