תשובת הראש בדיני מנהגים

There are those who claim that the reason why certain גדולים were seemingly punished for trying to reinstate daily ברכת כהנים was because it is אסור to change a מנהג.

There is a series of תשובות written by the ראש in כלל נה regarding the תקנות of the Jewish community in Toledo where he lived.

Specifically, one section of the series (אות י) deals with the rules of מנהגים, namely, which are legitimate and which are not as much, from which we see that there is such a phenomenon in הלכה as מנהגים that have a weak halachic basis and should be changed.

The ראש writes as follows:

All the מנהגים about which the חכמים said to follow the מנהג all are all סייגים, for example:

There is an איסור to do מלאכה on ערב פסח after חצות. Additionally, there is a מנהג not to do מלאכה before חצות either (see פסחים נ-נד for a number of other such מנהגים). 

But if a place has a מנהג that involves an עבירה, one should change the מנהג, even if it was established by גדולים, because one בית דין cannot uproot something from the תורה except בשב ואל תעשה. Even a נביא we don't listen to unless he has already been proven a legitimate נביא, like אליהו who brought קורבנות on הר הכרמל (see מלכים א פרק יח) to strengthen and reinforce שמירת המצוות.

Not only a מנהג that involves an עבירה should be changed, but even a מנהג that was originally instituted as a סייג could end up harming more than helping, in which case one should be מבטל the מנהג.

Now, there is a principle that where the הלכה is רופפת בידך follow the מנהג. However, this only means if and where there is ריפיון to begin with. Meaning that if the פסק הלכה is unclear in a given situation, then follow the מנהג, because then the assumption is that גדולים understood the הלכה this way and they therefore instituted the מנהג based on their understanding of הלכה. But where there is no ריפיון in הלכה to begin with, it is wrong to follow a מנהג that is against הלכה.

This is the דין of מנהגים in the realm of איסור והתר.

The ראש then goes on to discuss מנהגים relating to monetary matters. These are beyond the scope of this discussion.