Being he is using this with a miter saw, couldn't it be that the relay contacts are burned out? Usually with a miter saw they are on and off more than, say a table saw or jointer. So the relay contacts see more use over time.You are absolutely sure the relay is faulty? If you have a good ohmmeter you may want to isolate the relay and measure the coil resistance. Coil resistances are listed in the relay data sheet. Since these controller units allow the vacuum to run a short time after power is removed from the load I am guessing there are some other electronics involved.