Water gushing out of anti-siphon valves

Hi,

I have 3 zones in my garden.
Zones 1 and 2 were working fine, but when I turned on zone 3, the sprinklers work fine but water is also gushing out from under the cap on the anti siphon valve.
I have swapped the diaphram with zone 1 and the plunger and there is no change to zone 1 or 3.

Throughout all this, zone 2 has now started doing the same thing.

Any help would be much appreciated.

Thanks,
Mike