I have old Toro 254 inline valves (4 of them). I've discovered a leaking head a the lowest end of the line. I ordered replacement valves from SW and replaced the 4 diaphrams, seats, solenoids, etc. Is there anything that I could be missing that would cause this leaking?
That white plastic ring isn't actually the seat. It's a guide for the diaphragm. The seat is the part the diaphragm's washer presses against. The seat is attached to the body of the valve. It can't be replaced. You should take the valve back apart and check the seat for a tiny hairline crack or a pit. The crack can be very hard to see. Use a flashlight. You'll probably have to change the whole valve.