Was able to remove the diaphragm without damage using an exacto knife very carefully. Unfortunately, the o-ring dimensions I got from Watts were wrong. I put the top back on, following instructions. When I opened the inlet valve (with outlet valve closed) water gushed out the relief valve dump. I removed the top and tried again. This time, no water from the relief valve. Could this have been caused by air in the valve? I did not open any bleed screws either time. I did notice the top vent leak slowed to almost nothing while the sprinkler was running, but still dripped when the sprinkler was off.
So, called Watts and this time got dimensions for the diaphragm stem piston o-ring which agreed with the rough dimensions of the old o-ring. I'll try for the third time with a new o-ring. There was some green buildup in the o-ring bore which I tried to clean out but did not get all of it.