Some common repairs for a leaky bathroom faucet include replacing the washer in compression faucets, the O-ring in compression and cartridge faucets, and the cartridge valve in the cartridge faucets. Sometimes, the valve seat of the faucet may also need a replacement.
Washers in faucets act like a waterproof seal between the valve seat and the faucet stem. The O-ring acts like a seal for the metal parts within the faucet handles. Both of these tend to get damaged or loose with use and corrode over time.
To replace washers and O-rings in faucets, unscrew and remove the faucet's handle. Remove the retaining nut exposed to detach the faucet's stem. Remove the nut or screw that keeps the washer in position, remove it and replace with a new one. To remove the O-ring present around the faucet's stem, cut it with a knife. Replace with a new one, apply heatproof and waterproof grease, and reassemble the faucet parts.
If the surface of the valve seat inside the body of the faucet feels gouged, remove it with a wrench and replace it with a new one. Use replacement parts that are a perfect match for the original ones for a successful leak repair.
If the faucet continues to leak after repair, replace the entire faucet.