My irrigation sustem is over 10 years old,minor problems till this winter.I have a centrifical pump on top of a galv.tank w/a pressure diaphram connecting pump to tank. Am loosing prime while pump is running, somtimes zone 1,then other zones. do hear air/w water at pop-up heads,and some rotators, not all. when pump is not running I keep a static 50 lbs. of pressure at the gauge.I do have ck. valves at the input and output, also sprayed ALL connections with windex, this ck'ing for leaks,found none. This has been avery good system till now.Am looking for any advise...thanks in advance. Ed