Gemara

(Rabanan of the Beis Midrash): One who milks is liable for Mefarek (extracting).

143b (Mishnah): We may not squeeze fruit to extract the juice. if the juice came out by itself, it is forbidden [on Shabbos].

R. Yehudah says, if the owner planned to eat the fruit, the juice is permitted. If he planned to extract the juice, it is forbidden.

144b (Shmuel): One may squeeze grapes into a pot, but not into a bowl.

Inference (Rav Chisda): One may milk a goat into a pot, but not into a bowl.

He considers a liquid that goes into a solid like a solid.

145a (Rav): One may squeeze pickled vegetables for their sake, but not for the liquid. B'Di'eved, he is exempt.

One may squeeze cooked vegetables even for the liquid.

(Shmuel): One may squeeze pickled or cooked vegetables for their sake, but not for the liquid. B'Di'eved, he is exempt.

(R. Yochanan): Whether they are pickled or cooked, one may squeeze for their sake, but not for the liquid. B'Di'eved, he is Chayav Chatas.

Question (Beraisa): One may squeeze pickled vegetables needed for Shabbos, but not for Motzei Shabbos;

One may not squeeze olives and grapes. B'Di'eved, he is Chayav Chatas.

Answer (for R. Yochanan): It permits squeezing pickled vegetables needed for Shabbos, but not for Motzei Shabbos. The same applies to cooked vegetables;

This is for their sake, but not for the liquid. B'Di'eved, it is as if he squeezed olives and grapes. He is Chayav Chatas.

(R. Chiya bar Ash, and a Beraisa): Mid'Oraisa, one is liable [for squeezing] only for olives and grapes.

Rishonim

Rif (60b): What comes out of olives and grapes on Shabbos is forbidden. Therefore, we must say that Shmuel taught about Yom Tov. Bahag brought this law only regarding Yom Tov. Some say that Shmuel discusses Shabbos. The Mishnah discusses squeezing into an [empty] bowl, so they are squeezed for liquids. One may squeeze into a pot [of food], like Shmuel. R. Chananel says so. However, he rules unlike Shmuel and Rav Chisda. Rather, he rules like R. Yochanan, that one may squeeze pickled or cooked vegetables for their sake. One may not squeeze for the liquid. He is liable, like for olives and grapes, whether he squeezes into a bowl or pot. This shows that the Halachah does not follow Shmuel, and not Rav, who permits only into a pot. Some establish Rav and Shmuel to discuss Yom Tov, for Rav Chisda learns from them to permit milking into a pot, but not into a bowl. If so, the Halachah can follow both of them (R. Yochanan and Shmuel). This is wrong. Even on Yom Tov, one may not suckle straight from the udder, and all the more so one may not milk. Rather, they discuss Shabbos. Rav Chisda discusses Yom Tov, and the Halachah does not follow him, not on Shabbos and not on Yom Tov. We have a tradition from our Rebbeyim. All this is from R. Chananel.

Rebuttal (Rif): R. Yochanan obligates one who squeezes for their water, i.e. into a bowl. He argues with Rav and Shmuel only about Isur and liability, but not about distinguishing a pot from a bowl. Also, we find that they argue only about pickled or cooked vegetables, but not about grapes. One who says that they argue must bring a proof. Even though olives and grapes are squeezed for their juice, whether into a pot or bowl, into a pot is exempt, since the juice goes into a food. This is not like squeezing liquids, rather, separating a food from a food, which is permitted. Really, the Halachah follows Shmuel regarding squeezing grapes into a pot. R. Yochanan does not discuss squeezing grapes at all. Suckling from an animal is more stringent than milking. Even though suckling with the mouth is Mefarek k'Le'acher Yad (unskillfully), it is like squeezing a liquid. Milking into a pot is like separating a food, therefore it is permitted. All agree that a liquid that goes into a food is like a food. This shows that the Halachah follows Rav and Shmuel, and Rav Chisda. Rav Hai Gaon says so. There is no doubt about this.

Rambam (Hilchos Shabbos 21:12): Mefarek is liable due to Dash. One who squeezes olives and grapes is liable for Mefarek.

Rosh (22:2): One may not squeeze olives and grapes. If the juice left by itself, all forbid, whether they were destined to be eaten or squeezed.

Poskim

Shulchan Aruch (OC 320:1): One may not squeeze olives and grapes.

Beis Yosef (DH Tutim): Rashi explains that squeezing Peros is Mefarek, a Toladah of Dash.

Kaf ha'Chayim (1): One is liable mid'Oraisa even without a Kli.

Kaf ha'Chayim (2): Also one who squeezes raisins is liable.

Ashrei ha'Ish (2:28:1,3 citing ha'Gaon R. Y.S. Elyashiv Ztz"l): One should be stringent about citrus fruits destined to be squeezed for liquid, like for olives and grapes. One may not cut them on a plate, for juice comes out. One may cut on a table, for then it is not considered squeezing, for the liquid is wasted. We find that one may milk if the milk immediately goes to waste.

Lamed Tes Melachos (Dash 3E a:2): One who cuts grapes should cut them on a napkin, so the liquid will immediately be absorbed and be wasted.

Shulchan Aruch (ibid.): If the juice came out by itself, it is forbidden even if they were destined to be eaten.

Kaf ha'Chayim (4): The juice is forbidden until Motza'ei Shabbos. This is a decree lest one squeeze l'Chatchilah.

Rema: Only squeezing is forbidden. One may suck with his mouth the juice from grapes, and all the more so from other things.

Taz (2): The Beis Yosef brought from Shibolei ha'Leket that it is not normal to squeeze with the mouth. Chachamim did not decree about anything abnormal, and it is not a Toladah. The Beis Yosef says 'but sucking is forbidden l'Chatchilah.' It seems that this is a printing mistake. The stringent opinion is the Yere'im, who says that perhaps one is liable for sucking liquid absorbed in meat or bread, and all the more so it is forbidden for olives and grapes. In 319:16, we permit drinking through a cloth, like the Rosh. What one does with his mouth is not Borer. Likewise, here it is not Dash. (There, the Rema did not mention an argument.) Why does he bring the stringent opinion here? I did not see anyone stringent like the Yere'im. However, the Poskim agree that one may not squeeze liquid from meat.

Magen Avraham (3): All the more so one may suck other things, e.g. meat or bread that absorbed gravy or wine.

Rema (ibid.): Some forbid sucking with his mouth the juice from grapes and similar things.

Beis Yosef (DH Tutim): Hagahos Maimoniyos (21:10) brings from the Yere'im that one may not suck grapes or similar things with his mouth. This shows that it is called squeezing. One may not suck sugar cane on Shabbos, for it is normally squeezed for its liquid, to make sugar. Also, through sucking it, the juice is squeezed into his mouth. It is no worse than berries and pomegranates. However, Shibolei ha'Leket says that sucking with the mouth is not called squeezing. If so, surely it is permitted.

Magen Avraham (4): This opinion forbids even other things. The Bach says that one is almost Chayav Chatas for sucking liquid absorbed in bread or meat, since it was initially a liquid. Letter of the law one may suck grapes, for this is Derech Achilah, but it is good to be stringent. The Rema connotes that other matters are more lenient. Rashi says that cooked vegetables are more lenient than olives and grapes.

Mishnah Berurah (12): This opinion holds that other matters are no more lenient than suckling, which is also through a Shinuy, and even so Chachamim forbade it. Eliyahu Rabah is stringent only for olives and grapes, to which squeezing applies mid'Oraisa. This applies similarly to suckling, i.e. the fruit is outside his mouth. If he puts it in his mouth, all permit, for this is Derech Achilah.

Kaf ha'Chayim (12): One should be stringent about olives and grapes, and meat or bread that absorbed liquid. One may be lenient about other Peros.

Chayei Adam (14:6): If something fried absorbed much lard, one may squeeze it. It was initially a food, and also now it is a food. However, if meat absorbed liquid, one may not squeeze it for the sake of the liquid.