Bad gasket, debris between gasket and housing, warped plate, warped housing, bad hose, bad hose clamp, bad input shaft seal, bad O ring. Any one or all could be the prob. I eventually traced mine to a bad shaft seal. A few dollars from
http://www.marinepartsexpress.com/ and all was good.
Try taking the pump back off, fill it with water and spin the pulley (slowly cause water will shoot up in the air

). You might get lucky and see where the water is escaping. Other than that, I would take it apart again, use some light grit sandpaper to clean the mating surfaces, make sure there are no burrs where the hoses connect, and try again.