Are you allowed to use a plunger to unclog a toilet on shabbos?
Concerning a toilet, some rule that it is permitted to use a plunger, because it is very easy to clear the toilet (see Minchas Yitzchak 5:75). For a toilet, there is special reason to be lenient, because it involves an issue of kavod ha-brios. However, the Minchas Yitzchak writes that one should preferably use a non-Jew, or his weaker hand.
The Be’er Moshe is also lenient, explaining that the case is different from the case of the gutter, in that one needs only to push some of the blockage through (like pushing cocoa through a blocked straw). This is also stated by Binyan Shabbos, ps. 18, 303, in the name of Rav Shlomo Zalman Auerbach.
However, Rav Moshe Feinstein (Iggros Moshe 4:73) writes that one may only unclog a toilet with a plunger if it is partially stuffed, and water can still flow through (it is then permitted to use the plunger if it can be cleared with one or two pushes). He adds that if a toilet is regularly clogged, it is permitted to clear it, because this is not considered fixing. However if the toilet is totally stuffed, and this does not occur regularly, then it is considered a maaseh uman (expert labor) to clear it, and is forbidden on account of uvda de-chol. If there is a great need and an issue of kevod ha-briyos, one can clear the toilet by means of a non-Jew.
Because clearing the toilet is a pressing issue and involved kevod ha-beriyos, one can rely on the lenient opinions.