I have a similar problem with brine tank overflowing with my ERR3000 unit. Water is leaking externally from the top of the resin tank along the outside of the resin tank into the brine tank. After 24 hrs the brine tank will be full and start to overflow out the tank overflow hose. I traced the leak to the connection between the control valve and resin tank. Water is trickling down at the clamp retainers between the two half clamps that clamp the control valve and resin tank together.
What is the cause of the leak? O-ring seal worn out? There are three o-rings in there, should I replace them?