Question
installed a new waterpump everything seems to be right.  it will pump up to 50 psi and hold it until you turn the water on  then it will go down to around 15 psi will not get back  up to 50 until you turn the water off.  PLEASE HELP  

Answer
I sounds like there is too much water being used by the system for the capacity of the pump.  First, check to see if there are any leaks or open valves.  If not, and the system is running normally, you may have too many sprinklers on each station for the capacity of the pump.