BRS Dual Reactor leaking around cannister neck.

[Rover88]

I'm a little confused by this one. The dual reactor BRS system I have is leaking around the threading that the canister uses to attach to the plastic receiver. The O-ring is in perfect shape, I cleaned it thoroughly as well as the channel it rests in and the lip it seats on. When I thread it in I tighten it up until I can't anymore. Once the pump is turned on and the cannister fills, you can watch the water slowly work its way around the threading, then start to leak. The O-ring isn't sealing in either cannister. Is there a trick to this?