Eating and drinking prior to the visit:[1]

Some Poskim[2] rule one is not to eat or drink anything prior to visiting the gravesite of a Tzadik or of a relative. [Thus, starting from Alos of that day, until after the visitation, one is to fast.] Others[3] rule one is to have a small snack prior to visiting a gravesite, although is not to eat a full meal.[4] Practically, the Chabad custom is not to eat any food prior to visiting a grave site, although one is to be particular to drink before visiting.^[5] [The Rebbe was particular about this custom and that others should follow it.[6] On the other hand, there were instances that Rebbe and Chassidim ate something small prior to visiting.[7] The Rebbe was particular not to visit Kevarim on a public fast day, being one is unable to drink beforehand.[8]]

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[1] See Gesher Hachaim 1/29; Nitei Gavriel Aveilus 2/656; Chikrei Minhagim 1/273; Hiskashrus 703 footnote 25 and 885

[2] Alef Hamagen 581/109; Gesher Hachaim 1 29/4 in name of Maaneh Lashon; Yalkut Avraham 581; Luach Dvar Yom Beyomo; Likkutei Tzevi Kol Bo p. 244

[3] Custom in Alef Hamgen ibid; Yalkut Avraham 581

[4] The reason: Fasting is problematic, as it appears similar to the forbidden procedure of arousing the dead [Doreish El Hameisim] which consists of fasting and then sleeping overnight near the grave. [Sanhedrin 65b] Thus, to avoid this issue, the above Poskim write that some are accustomed to taste something before they enter but not to actually eat a meal. [ibid]

[5] Sefer Haminhagim p. 168 [English]; Igros Kodesh 3/279; 9/301 [regarding Kivrei Avos- See Hiskashrus 885]; 24/263 [brought in Shulchan Menachem 5/327]; Igros Kodesh Rayatz 6/282; See Hiskashrus 703 footnote 25 regarding the

Custom of Rebbe Rashab: The Rebbe Rayatz records that when the Rebbe Rashab visited the Ohel of the Baal Shem Tov in Mezibuzh he was fasting. [Igros Kodesh Rayatz ibid; See also Likkutei Dibburim 4/37 p. 1354 that the Rebbe Rayatz was fasting when he visited the Ohel] The Rebbe commented on this that it requires further analysis if this is a directive for the public being that he was told by the Rebbe Rayatz to drink.

[6] The Rebbe once told a Bochur who ate prior to visiting the Ohel that he needs mercy/Rachmanus. [Hosafos to Sichas Kodesh 5710 p. 226] In Igros Kodesh 24/263 the Rebbe implies that if one does eat beforehand, it is better that he not visit the Ohel.

[7] Rav Binyomin Klein said that there were times that the Rebbe ate chocolate prior to visiting. [Hiskashrus ibid footnote 25]; Rabbi Leibal Groner related that Rabbi Kalman Marlow A”h ruled that one may eat foods with a blessing of Shehakol prior to visiting. [Rabbi Yossi Marlow Sheyich’ was unable to confirm this statement]; The Rebbe Rayatz ate Mezonos prior to visiting the Kever of the Arizal and Kever Rachel. [Hiskashrus ibid] See Hiskashrus 412  

[8] Nitei Gavriel 86 footnote 7