I learnt it was OK to answer amen after each part of Bircat Kohanim even if no Cohen is present. I was told this was wrong. Please clarify this issue.
The Beit Yosef at the end of Ch. 127 quotes Rabbi David Abudreham (p. 116) who says that the Rif (Rabbi Issac Alfasi) was asked whether the congregation should answer Amen when the cantor says Bircat Cohanim (priestly blessing). He replied that one does not need to answer Amen, only when the Cohen raises his hands for priestly blessing. The Abudreham adds that the reason is because we do not answer Amen, only when we hear the blessing from the person blessing. Therefore there are some individuals who answer “Ken Yehi Ratzon” (May it be [G-d's] will) instead of Amen. The Beit Yosef adds that also the Rambam agrees that if they (the congregation) do not have a Cohen at all, the Cantor says Elokeinu V’Elokei Avotenu… when he gets to Sim Shalom (Grant peace – the final blessing of the Shmone Esreh prayer) and the people do not answer Amen. The Darkei Moshe (ibid 4) adds that the custom is to say Yehi Ratzon and not Amen, and in Mahara it says that he didn’t say anything. The Shulchan Aruch (Orach Chaim 127, 2) rules that if there are no Cohanim, the Cantor says Elohenu V’elohei Avotenu Barchenu Bavrachah Hameshuleshet… V’Ani Avarechem, and the congregation does not answer Amen but Ken Yehi Ratzon. The Mishnah Berurah (ibid 10) explains the reason, that Amen is relevant only when answered after a Cohen blessing, but not when a cantor is saying it as a request that G-d may He be Blessed should bless us with the blessing the Cohanim bless. The Kaf Hachaim (ibid 16 17) quotes at length the opinion of the Poskim that one should not answer Amen but Ken Yehi Ratzon. He adds that however Responsa Yachin U’voaz (25) writes that there are some who do answer Amen and that both customs are OK. But the Derishah writes that there is a prohibition because one should not answer Amen in vain. Rabbi Moshe Shternbuch writes in Responsa Teshuvot V’Hanhagot (vol. 2, 101, vol. 3, 43) that Rabbi Chaim of Brisk ZT”L conducted himself like the Mahara in Darkei Moshe not to answer at all, and his reason is because there is an Aseh prohibition (positive-obligatory commandment) for a non-Cohen to ascend the synagogue rostrum and bless the congregation. And if he will answer Amen on the blessing of the cantor it may seem like he is accepting it as a priestly blessing, and that is an Aseh prohibition as explained in Ketuvot 24b see Rashi there, (if it is intentional there may be a prohibition of “Bal Tosif - do not add”), and he feared to accept the blessing out of concern of that prohibition. Therefore he avoided answering the cantor’s blessing all together. The Mishnah Berurah (ibid 11) explains the words of the Shulchan Aruch and says that the Ken Yehi Ratzon is said at the end of the third blessing as it is all one issue, that the cantor includes in his prayer what the Cohanim say. And others answer Ken Yehi Ratzon after each blessing. The Aruch Hashulchan (ibid 4) writes: By us the custom is to say it after each blessing, between every verse and verse. In conclusion: Some answer Amen after the blessing of the Cohanim which the cantor says. Others do not answer at all as they are concerned with the prohibition. Nevertheless according to the opinion of most Poskim, and as far as I know this is the custom in most Ashkenazi and Sephardic communities, one does not answer Amen but Ken Yehi Ratzon after each verse of the priestly blessing recited by the cantor.