I always put a comma before the word but, simply because right or wrong, I understand this to be correct. However, sometimes I don't 'hear' the pause before the 'but' in certain sentences, and find myself asking if it should be there. Example: 'I checked the side mirror. The van was still there, but no longer seemed to be gaining.' I'm not hearing the pause before the 'but', so should it be there or not?