what would cause a sprinkler valve to leak at top of the assembly, a new sprinkler valve?

Question

i replaced two valves and they are leaking when timer kicks on and when they shut off

Answer 1

Faulty gasket, which could be cracked, or does not seat itself properly when engaging and disengaging.

Answer 2

THERE IS AN ANTI SIPHON DEVICE ON TOP CONSISTING OF A SPRING A ROUND PLASTIC PLUNGER ANS A GASKET
THE FUNCTION OF THIS IS WHEN THE WATER SUPLY FROM THE STREET GOES OUT THE LINE WILL SUCK BACK UP AIR AND NOT SITTING WATER LIKE FROM THE TANK OF THE TOILET OR THE INSIDE OF A WASHER MACHINE A SWIMMINGPOOL ETC,,, THERE ARE REPLACEMENT KITS SOLD FOR THESE VALVES AND ALSO HIGHER QUALITY VALVES USUALY MADE FROM BRASS AVAILABLE. TAKE THE VALVE APART CLEAN IT INSPECT THE PLUNGER THE GASKET AND THE SEAT REPLACE IF WORN OUT AND RUN AGAIN IF THE LEAK IS ON THE TOP OF THE SPRINKLER HEAD YOU DO NOT HAVE ENOUGH PRESSURE IN THE LINE TRY WATERING AT NIGHT.