Why this PVB is leaking ?

[kulkarnipb]

I replaced 3 , new from shop, but same issue again and again. I tried 1 inch, zurn (don't remember model number ) as well as watts (800m4qt) PVBs. What is wrong I'm doing here . Why all PVBs are leaking from top ?

By the water pressure is not more than 50 PSI.

 

[Geofd]

Sometimes the leak until the system is totally pressurized another thing I do is install a spring loaded check valve within a foot of the house side of the back flow device