May one unplug a drain pipe on Shabbos?
It is forbidden to unplug a stuffed drainage pipe on Shabbos unless lack of doing so will cause flooding to one’s house, in which case one may remove the blockage of the pipe with an irregularity.
Example: If one’s rain pipes on his roof became stuffed with dirt and twigs and is causing flooding in one’s house, one may use his feet to press down the blockage, allowing the water to flow freely.
May one unplug a stuffed toilet or sink?
Yes. One may even use a plunger or snake to do so. However some Poskim limit this allowance to only if the blockage occurs often and only if the sink or toilet is not completely stuffed. Others allow it without difference. If easily available it is best to unplug the sink or toilet through a gentile.
 As since this case involves loss, the Sages did not decree against fixing the pipes in an irregular way. [ibid]
 Beir Moshe 1/29; Minchas Yitzchak 5/75; 7/19
 Igros Moshe 4/40
 As only then is it not considered Tikkun Keli.
 Bier Moshe and Minchas Yitzchak ibid.
 Minchas Yitzchak 5/75; 7/19