Have a question about my head:

I have a Groco HF in my Catalina 34. It fills when the through hull is left
on.

I've rebuilt the head. Also installed new hoses and an anti-siphon valve.
The system is set up: Through hull intake to pump to Siphon valve to bowl.

Still, the bowl fills if the switch is left in the flush position. I thought
a an anti siphon valve would stop that.

What am I missing here?