problem with valves closing off

zepfan

We have a job where there are four B and G circulators model#80 bf 7.5000. On the inlet side is a 4" titan flow control gate valve.On the discharge side is a 4" titan flow control triple duty valve.The seal in one of the pumps is leaking, and needs replacement.The issue is when both valves are shut,the flow of water out of the boiler drain located below the triple duty valve's handle does not slow down at all.Obviously one of the valves,or both are not sealing.My question is that is there any know way to determine which valve is not holding,or should I assume they both are not and replace them both?The application is boiler water for space heating.Thanks to all