Orbit Watermaster Npt Automatic Antisiphon Control Valve 57224

Pros: I have used these valves for 15-20 years trouble-free.
Cons: Recent purchases have been defective or unreliable.

I recently purchased some new valves to replace older ones that no longer functioned properly. The new valves fail to consistently shut off . Sometimes they do and sometimes they don't. I have taken all trouble shooting suggestions and there is no improvement. Unless I can find a solution I am going to have to shop for another brand of valve. I have returned 4 valves and the replacements continue to malfunction.
I am operating on a line with a static pressure of 50# and all valves are more than 6" above the highest sprinkler head.
The only other factor I can think of is that most of the heads are downhill from the valves.
All valves are 3/4" thread and supply line is 3/4" PVC

Pros: Nice valve, reasonably priced and readily available.
Cons: Anti-siphon check-valve failures causes unnecessary valve replacement unless failure is understood and parts acquired.

This valve has a weakness (design flaw) in the plastic anti-siphon check-valve. This valve, called the "seal carrier" cracks about 1/8 inch in from the outer diameter causing the check-valve seal to no longer function. The result is that when the valve is turned on, water gushes out from this crack and comes out of the anti-siphon vent area of the valve. I have had three of these failures in a year. The water pressure is normal city pressure. Curiously, the service department has replacement kits readily available at (800) 488-6156. The kits include the "seal carrier" and the "Lid Top Gasket" as well.