Last time mine siphoned, there was alot of grass and small woody debris that collected at the vent. Pretty concerning, but it didn't clog. I never thought of having 2 vents, that's a good idea.
I suspect mine may have siphoned again last night, but I didn't have time before work to go inspect. Got just shy of 4" overnight. From the house I couldn't see any evidence that water had gone over the emergency spillway, I'll have to check it out this evening.
