If a sink or bathtub drain runs sluggishly even though you have cleared all surface hair and found no physical clog, the issue is typically caused by improper plumbing venting or a partial obstruction in the main branch drain line. When a roof vent pipe is blocked by leaves or debris, air cannot escape the drainage system, creating a vacuum that severely slows wastewater velocity across multiple fixtures. Verification: Check if other plumbing fixtures gurgle when you empty the tub, which indicates a venting problem.
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