I recently started to learn B'rachot again. I am making an attempt to write up notes. My goal is to be as clear as possible and to only write those things that I think are particularly interesting or insightful.
מסכת ברכות פרק א
א,א מאימתיי קורין את שמע בערבין: משעה שהכוהנים נכנסין לאכול בתרומתן, עד סוף האשמורת הראשונה, דברי רבי אליעזר. וחכמים אומרין, עד חצות. רבן גמליאל אומר, עד שיעלה עמוד השחר.א,ב מעשה שבאו בניו מבית המשתה, ואמרו לו, לא קרינו את שמע. אמר להם, אם לא עלה עמוד השחר, מותרין אתם לקרות.א,ג ולא זו בלבד, אלא כל שאמרו חכמים עד חצות, מצותן עד שיעלה עמוד השחר. הקטר חלבים ואברים, מצותן עד שיעלה עמוד השחר; כל הנאכלים ליום אחד, מצותן עד שיעלה עמוד השחר.א,ד אם כן, למה אמרו חכמים עד חצות--אלא כדי להרחיק את האדם מן העבירה.
Is Rabban Gamliel disagreeing with the Chakhamim? If so why does he discuss their opinion with his sons? Additionally, why do his son's not know his own position?
Rabban Gamliel does not seem to be disagreeing with the Chakhamim. Even though he is presented as a separate position it is clear from his discussion with his sons that he agrees with the Chakhamim. His sons knew that the mitzva of K"S was "all night". However, they were unclear on the intention of the g'zeira of "until midnight". In other words, they did not know whether the g'zeira was a simple prohibition or whether the Chakhamim had invalidated any K"S said after midnight. If it was a simple prohibition then even though his sons transgressed this g'zeira they could still fulfill the mitzva of K"S until day-break. However, if it was an invalidation than there would be nothing they could do.
Rashi explains that the purpose of this g'zeira is to encourage a person to not procrastinate and "forget" to say K"S. Both ways of understanding the g'zeira would accomplish this objective. However, it would be a far more affective "encouragement" if the Chakhamim actually invalidated any after-midnight K"S. This is what Rabban Gamliel clarifies for his sons (and what he meant in the first half of the mishna): the Chakhamim's g'zeira was only a prohibition not an invalidation.
Rashi does not appear to follow this approach. He writes on 3A (D"H:"Lav R' Eliezer Hi") that Rabban Gamliel does not have the s'yag ("fence") of the Chakhamim. It seems that according to Rashi Rabban Gamliel's sons did not know what their own father's position was. It is also unclear, according to Rashi, why Rabban Gamliel was so concerned with explaining the Chakhamim's position.
Rambam does seem to follow the above approach. He writes in the Peirush HaMishnayot that the halakha is like Rabban Gamliel. In the Mishne Torah (Hilchot K"S, 1:9) he writes that the mitzvah of K"S is until midnight but if one transgressed and read later - until amud hashachar - one fulfills his obligation. He explains that "they" only said until midnight to distance men from sin.