Valves on most pumps are made of a plastic cage with a metal seat. Sometimes the cage will break due to debris in the supply water, allowing the valve to float.  The metal valve seat could be worn or scored due to dirty supply water.  The valve assembly sits on an o-ring that fits between the valve assembly and the head of the pump.  Click here for a selection  of replacement valve assemblies.  Check this o-ring for wear and the brass seat for scoring.  Usually this condition is caused by cavitation due to insufficient water supply.  Cavitation in this area will destroy your pump. The worm holes under the o-ring are in the head of the pump and not fixable.  Fixing your water supply problem and replacing the valves and o-rings will help for a while but the problem will soon return as the pressurized water will work it's way under the o-ring, through the wormholes.  For a discussion on pumps click here.  To see a range of replacement pumps click here.