The short answer
A pop-up drain can leak at several separate seals. Dry the assembly, then test the bowl, flange, pivot rod, tailpiece, and P-trap in stages. Replacing the whole drain before locating the first wet joint can leave the real leak untouched.
Map the joints
Place a dry towel or tray below the sink. Wipe the drain body and trap dry, then locate the flange inside the bowl, the large gasket and locknut below it, the horizontal pivot rod and retaining nut, the tailpiece, and the slip-joint connection to the P-trap.
Run three small tests
- Bowl-hold: close the stopper and add a shallow amount of water without releasing it. Moisture below the flange points toward the flange or bowl gasket.
- Drain-flow: release the stopper while watching underneath. A leak that starts only during flow may be at the drain body, tailpiece, or trap.
- Pivot: move the lift rod while a small flow runs. Moisture centered on the pivot nut points to its seal or alignment.
Wrap dry tissue around one joint at a time to reveal the first damp spot. Water can track down the drain body and drip somewhere else.
Delta instructions show that pop-up assemblies use several distinct seals and require the pivot to face the faucet. The specified use of silicone, putty, gaskets, or thread sealant varies by drain model.
Repair the confirmed joint
A slip-joint washer may need reseating or replacement. A pivot seal must face the correct way. If the flange seal failed, the drain usually must be removed and reinstalled using its specified sealant. Do not overtighten plastic fittings.
Stop if the cabinet is swollen, water is near wiring or an outlet, the sink is cracked, or a corroded part will not move safely. Retest with the cabinet empty and dry.