When I create a custom back button, I use the following code:
UIBarButtonItem *leftButton = [[UIBarButtonItem alloc]initWithTitle:@\"Yeah\" style:UIBarButton
UIButton * backButton = [UIButton buttonWithType:UIButtonTypeCustom];
[backButton addTarget:self action:@selector(popViewController) forControlEvents:UIControlEventTouchUpInside];
[backButton setFrame:FRAME_DEFINE
[backButton setContentHorizontalAlignment:UIControlContentHorizontalAlignmentLeft];
[backButton setExclusiveTouch:YES];
[backButton setImage:[UIImage imageNamed:BACK_BUTTON_DEFAULT_ICON] forState:UIControlStateNormal];
[backButton setTitle:@"BACK" forState:UIControlStateNormal];
[backButton setTitleColor:[UIColor blackColor] forState:UIControlStateNormal];
UIBarButtonItem *backMenuBarButton = [[UIBarButtonItem alloc] initWithCustomView:backButton];
self.navigationItem.leftBarButtonItem = backMenuBarButton;