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There is a small hole facing up in the sewer pipe that is the hose connection of the sink overflow, if the sink does not have an overflow, the small hole in the sewer pipe is useless and can be plugged.
It can be used when it is blocked, and it is generally used at the bend of the pipe or when the pipe is particularly long. Unscrew it and use a tool to pass it through. Generally, it is not necessary.
Such a leak can be repaired if the handle of the faucet can be disassembled, and it is a broken rubber pad inside.
That's right, you go and install one with an adapter, and then you can install the washing machine and the washbasin at the same time.
It should be that there is hair and other debris in the sewer pipe, which sticks to the wall of the sewer pipe. It is advisable to clean the sewer pipes and the situation will improve. Good luck.
It is estimated that it is caused by the large change in the diameter of the front and rear pipes, the thickness of the front section and the thin back section, and the high pressure of the water.