If your sink backs up or drains sluggishly despite having a clean P-trap, the restriction is typically located further down in the main branch drain line or caused by a blocked roof vent stack. When debris blocks the roof vent pipe, air cannot escape the drainage system, creating a vacuum that severely impedes wastewater flow across household fixtures. Another common cause is an improperly adjusted pop-up stopper acting as an internal dam. Verification: Check if other plumbing fixtures gurgle or back up when the sink fills with water.
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