Line Buffered Cat

I'd like to watch the output of a UART device in Linux, and I only want to see the content when there are a whole line. So I prefer some kind of line-buffered cat such as:

$ cat --line-buffered /dev/crbif0rb0c0ttyS0

But unfortunately, cat doesn't have a line-buffered option. And fortunately, GNU grep has such an option. So we can do

$ cat /dev/crbif0rb0c0ttyS0 | grep ^ --line-buffered

Since every line has a ^ (line start), so each line matches the grep. Note that I ever tried

$ cat /dev/crbif0rb0c0ttyS0 | grep . --line-buffered

But this does not work. Only empty lines are printed, and I don't know why...