Jpsdad
Option #2 seems to be the winner. I had a new pond builder come out and he replaced the one inch vent with a 2 inch. The siphon has gone through multiple cycles and is working properly.
Still no explanation of why this became a problem 4 years after the pond filled. But I am very happy the problem is fixed and the pond seems to have recovered.
Nehunter you were also right on target
Thanks for all the input of forum posters.
![[Linked Image from forums.pondboss.com]](https://forums.pondboss.com/ubbthreads.php?ubb=download&Number=16222&filename=20200529_204242.jpg)
Cliff