How to: Inject Fernox Express via the Worcester Bosch drain port 

Hi Wayne, there is residual chemical after the injection process, you just have to clean with tissue. It does not continue looking after shutting the drain valve. If it did, then you have a faulty drain off valve, in which instance, you would have to replace at some point. In all our time opening and closing this drain valve, we have yet to come across a leaking drain valve, although not impossible as it is a mechanical part. JK