WORN SEALS

If your pump is leaking water from underneath, between the head and crankcase, you likely will need to replace the plunger seals.  

These come in a kit with the various seals and o-rings that make up a packing kit.  Kits usually come in sets of  3 packings so you can do all the pistons, at once, which is a good idea. See the discussion on broken or worn plungers as well.  

Tell-tale signs of worn seals or broken pistons is water in your crankcase oil.  Check the oil for a  milky color.

Most pump manufacturers have a similar design for a packing.  It usually includes a high pressure seal, low pressure seal, a couple of plastic rings and a few o-rings. 

These seals are not difficult to replace, just make sure you get them in the right order.  Check here for servicing information from various pump manufactureres.

When replacing packings it's important to check the pistons.  Small cracks in pistons will not only leak water but will quickly wear out a new set of packings. See additional discussion on broken or scored plungers.