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LASALLE BRISTOL - 1-1/2” ABS Vent Check 2 breaks the siphoning action caused by water draining. The sealing disc inside vent check automatically opens and closes allowing proper draining without septic gas escaping. The 1-1/2” Vent Check 2 is designed to meet current plumbing standards and is interchangeable with old style vent checks. Vent Check 2.