I have a five valve irrigation system. Four of the five work perfectly. One, when it "turns on" (in sequence, it is the third zone of the five) knocks/hammers unstoppingly until I shut off the zone. Since the system is at least 15 years old, I suspect that particular valve is the problem. I can easily enough replace either the solenoid or the diaphragm (or both). Before I do that --a couple questions --(1) Which of the two is more likely, so I can do that one first? (2) Will replacing those two (one at a time) solve the problem? (3) Or am I on the completely wrong track? (4) A friend (with no particular expertise) thinks I may have a complete break somewhere in the zone causing the problem --maybe?