If your exhaust valve is leaking you can usually hear it through the barrel, often a good cleaning can fix the problem but the valve could also be damaged or the body. On the old style 880 I often have a problem getting a good seal on the bottom of the pump tube, sometimes this is due to wear on the pin holding the tube in place. The holes will wear and not hold the pump tube down far enough. Unfortunately the easiest fix for that is a new pump tube, and they are only available used.