Yes, the avos kept shabas all week long, but the imahos did not. Consider:
- In B'reshis 14:22, Avraham refuses to take money, telling the king of S'dom to keep it for himself and give it to others. Of course, this was because Avraham couldn't take money on shabas (i.e, every day).
- In chapter 18, Avraham brings food to his visitors, but only Sara and "the young man" prepare it.
- In 27:9, Rivka tells Yaakov to bring goats, but offers to prepare them, herself, for eating.
While you can ask how Avraham cut animals in half (15:10), this was presumably on a direct command (not stated in the pasuk) from Hashem, which of course overrides a general mitzva.
And his "repaying his credit" (Rashi to 13:3) was done not with money but with services, which can be done on shabas: he was a magid, an itinerant preacher, who of course can work on shabas.
You may ask about Yitzchak's digging wells in 26:18, 22. But we see in verses 19 and 21 that his servants did the digging. The two interior p'sukim specifying it was his servants surrounded by the two saying "Yitzchak" serve as a k'lal ufrat uchlal, i ata dan ela k'en hap'rat, so we see that all the wells were dug by people affiliated with Yitzchak. None, however, were actually dug by him: what do you take him for, a m'chalal shabas, chas v'shalom?