Did Muslim Africans brought to America as slaves keep their faith?

Yes and no. There are definitely evidences that Muslims enslaved from Africa tried to keep their faith. A great example is the Bilali Document, a handwritten work on Islamic law based on the Risalah of Abi Zayd of al-Qayrawan (a famous scholar of the Maliki guild of Islamic law). The document tries to adapt basic Islamic law (how to pray, how to give the obligatory charity, etc) to the reality of living as a slave in America.

With that said, as far as I know, there is no evidence of this being successful for more than a generation or two. In other words, they may have kept their faith and some may have passed it down to their children, but not much more than that. The resurgence of Islam amongst the Black community in America that was seen in the 20th century was not a direct continuation of enslaved Africans passing down their faith until the modern era.