I have a system that we installed 3 years ago that has been experiencing teribble water hammer all summer. The system never did this before. There is a 3/4" copper main line that is T'd inside the house & exits the front & back. There are 2 master valves feeding 1" mainlines which feed 12 zones with 1" laterals (all PVC). All valves & heads are Rain Bird & the system runs @ 100psi. We have already: Replaced both master valve diaphragms, installed 2 water hammer reducers on the copper inside. Neither of these did a damn thing to stop the water hammer. My next 2 options are: Replace all diaphagms (but why? the system is only 3 years old!), Or add SAM fittings to every head to prevent the water from draining. Has anyone out there had similar problems? If so, I'd appreciate your input. This lady has been a saint (I would have been really ticked). She has a newborn baby & has had to listen to an AK-47 in her basement every other day at 6:30 in the morning.