The zoeller we use, the # of which I don't recall off hand, says right in the book that the vent hole is integral to the pump and as such not needed. Where I used to work we never drilled a vent hole regardless of the pump and I cannot recall a single instance in 17 years of being called back for or coming across an air locked resi sump pump.
Without looking, it's a 3/16 hole. When I install sump pumps I drill the hole 1" above the top of cast pump. Never had a problem.
I drill it even if I don't use a zoeller. Cheaper brands sometimes don't mention drilling a hole. I say **** if and drill it.
I call bull**** on that one.I catch dewatering pumps constantly air locked, basement flooded with pump running sometimes when I arrive.
It takes one time where the water drops below the impeller area, a quick shot of movement that creates an air bubble in the area of the impeller and that pump will never force the check valve to open.
You've been very fortunate, and I always drill those pipes knowing there's a relief hole on the pump itself.
It's right in the manufacture's instruction on all zoeller products pertaining to submersible pumps, whether dewatering or macerating.
You obviously haven't seen airlocked pumps. Pumps will not pump air and anytime the air around the impeller can't escape then the pump will run and do nothing. I have drained deep commercial ejector pits more than once to clean and turned the pumps back on only to find they are airlocked because there is no hole in the discharge. (A simple fix on a deeper pit is keep filling it usually 5-6 ft of water will compress the air enough to get water in the impeller.I call bull**** on that one.