Originally Posted by gljurczyk
Herk why can't you just add a 1/2" back flow preventer?
You can; BUT ---
Vacuum breaks are normally used in this application (AND dishwasher discharge lines for the same reason) to create an air path into the black tank should there be a negative (suction) pressure during the dump cycle.
Without the vacuum break (ie using a check valve like some cheap after market "turbo flushers" include), the dump negative pressure can "flutter" the check valve (ever watch the water flow at the dump outlet using a clear elbow?) potentially allowing bacteria on the dirty side of the valve to enter the fresh (campground side) water source.
Backflow preventers are "inline" and Vacuum breaks are always an "elbow" with a hat.
ALSO note that the inline backflow preventer dumps its pressure through the outlet holes! If this is INSIDE your camper there will be water everywhere.
(NOTE: NOT a plumber; but this is how I understand it.)
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