WATER LEAKS FROM BOTTOM OF PUMP

Any water leak from the pump or pressure hose and fittings will cause pressure drop.  Pressure is produced at the nozzle.  It's the amount of water being pushed through that hole which creates pressure.  Less water = less pressure.  Check to make sure you are not losing water from anywhere downstream of the pump.  

Quick connectors are notorious for leaking water.  Check them and regularly and replace the o-ring if leaking.  Click here for selection of replacement connectors and o-rings.  

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.  

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.  See additional discussion on water in oil.