Eating and drinking prior to the visit:
Some Poskim 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 rule one is to have a small snack prior to visiting a gravesite, although is not to eat a full meal. 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. [The Rebbe was particular about this custom and that others should follow it. On the other hand, there were instances that Rebbe and Chassidim ate something small prior to visiting. The Rebbe was particular not to visit Kevarim on a public fast day, being one is unable to drink beforehand.]
 See Gesher Hachaim 1/29; Nitei Gavriel Aveilus 2/656; Chikrei Minhagim 1/273; Hiskashrus 703 footnote 25 and 885
 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
 Custom in Alef Hamgen ibid; Yalkut Avraham 581
 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]
 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.
 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.
 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
 Nitei Gavriel 86 footnote 7