When the undermount bathroom sink starts to recede or will not drain, it is necessary to find the obstruction and unclog the sink. The first thing you should do is lift the drain plug and clean it. Cleaning the hair and dirt around the plug most likely will solve the problem. If you pull the plug and the water does not drain, there are other steps you will have to take. Put a rag in the overflow opening in the back of the sink. The overflow hole is usually under the faucet on the back of the toilet bowl. Place the edge of a cup plunger over the drain hole. Push down and then pull up on the plunger handle without lifting the cup out of the drain hole. Do this three times and then lift the plunger from the sink.
Place a bucket under the sink of the trap (the curved piece of the drainpipe) under the sink. Some sink traps have a square plug that can be removed with an adjustable wrench. Pull the plug away from the sink trap and use a small flat-tip screwdriver to pull the debris out of the trap. Disconnect the sink trap from the pipe if you do not have a cleaning plug. Place the bucket near the sink trap and drain pipe connection. The drain tube is the straight tube that comes from the bottom of the small undermount bathroom sink. Loosen the sliding nut that connects the sink trap to the drain pipe with a pair of extension tongs. Standing water in the sink must leak around this connection in the bucket.
Once the water flowing from the sink has stopped, place the bucket under the connection of the sink trap pipe and the wall. Using the extension tongs, loosen the sliding nut that connects the sink trap to the pipe wall. Pull the large undermount bathroom sink trap away from the plumbing and empty the contents in the bucket. Take the sink trap to the bathtub and run the water through the trap to clean the inside of the trap. If the debris came out of the sink trap, the obstruction was inside the trap. A trap that was relatively clear may indicate an obstruction in the wall pipe. Buying or renting a snake from a plumber at a hardware store. A plumber’s snake looks like a coil spring with a handle. Insert the end of the coil of the snake into the wall tube.
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