I believe that the sequence that this question is asking about can be found in chapter 44. The chapter begins with Maniac Magee going on his early morning runs. As his runs continue, Maniac thinks that he hears another set of footsteps. It turns out that Mars Bar is also out on early morning runs. The two cross routes and/or find themselves running on opposite sides of the street. The two boys eventually end up running together. On one such day, they come across a very distraught Piper McNab. Piper explains what he and his brother were doing, and Mars and Maniac come to find Russell high up on the trolley trestle, frozen in fear. Piper begs Maniac to help, but Maniac can't do it and leaves.

Readers find out in the next chapter that Mars Bar ends up saving Russell. The action is a very selfless act out of a character that has shown to be very selfish and self-centered through much of the book. I personally think that a reader would have to answer that Maniac did influence Mars Bar to take action. I would hesitate to say that Maniac was the only influence, but I think Maniac's presence and actions in Two Mills played a part in transforming Mars Bar.

Readers are told that Maniac is a character that is "color blind" to the issues of race, and we repeatedly see that Maniac treats everybody fairly and with respect, regardless of how they treat him or of their reputation. Mars Bar might be antagonistic to Maniac at various points, but that absolutely means that Mars was influenced by Maniac. Fortunately, Mars Bar changes for the better because of his continued interactions with Maniac.