I've recently replaced the outlet flush valve on my close coupled toilet cistern and cannot get the backnut to seal. Water leaks by the nut into the pan causing the system to fill periodically(annoying). Due to the size of the backnut, I haven't been able to get it more than hand tightned, hence the problem. I don't want to by a large, expensive wrench for a one-off operation, so would It be acceptable to seal round the nut with some silicone sealant.