The air check is to prevent the control valve sucking air during the brining stage of regeneration not during the brine tank fill stage at the end. You should have a float assembly in the 4" round tube to prevent overfilling of the brine tank during a control valve malfunction exactly as you are experiencing now. If your softener is old enough it may not have the Fleck 2310 safety float and you should upgrade to that once the problem is solved. 1.If the brine tank overflows continuously then you have a problem in the 5600 piston assembly and possibly the brine valve. 2.If the brine tank overflows and stops them you have a problem in the injector, screen, and/or brine valve and/or the drain line from the 5600 could be clogged. The way it's supposed to work is... The last stage of regeneration is the brine tank fill. The control valve puts a volume of water into the brine tank based on the salt setting in the back of the 5600. That water dissolves the salt and waits till the next regeneration. When the next regen starts the 5600 sucks out the brine and flows it through the resin. If there's a problem and the 5600 doesn't suck out almost all of the brine the level of water will be higher after the last (brine fill stage). Each successive regen will add more water till the brine tank overflows. Questions... Water system or well water? How old is the softener? Has anyone been doing routine maintenance on the softener?

If you have an open clawfoot tub changing the waste & Overflow kit is not a big deal. On the other hand, if your tub is a standard built-in tub you may have a glue-in type wste & overflow and it could be quite expensive to change, depending upon whether it is readily accessible or whether it would require opening a wall or ceiling. Fortunately, in your case there is no need to change the waste & ovrflow piping under the tub. Rather than get another complete waste & overflow kit ask your plumbing supplier about a "Trim Out kit" in the finish you desire. With a trim out kit you will retain the existing piping under the tub and only the tub drain basket and overflow cover will be replaced. That can be done very quickly from inside the tub.

Merry Christmas - and thanks for your replies! :) I should probably explain that I have a very strong sense of smell - unnaturally strong - so this may be a smell that no one else notices. However, I have noticed weird smells coming from the sink overflow holes in 3 different houses. These were other people's homes, so I can't try the bleach trick to see if that fixes it. We live in a temporary rental house that was constructed 6 months ago, so the overflow holes haven't had a chance to start stinking quite yet. I primarily notice the smell when I bend down over the sink to brush my teeth or wash my face, but I've also smelled it when standing upright. It seems to happen mostly when the faucet is running, perhaps when the water pushes air out of the overflow hole (assuming that's possible). I've wondered the same thing about the cultured marble sinks - if code requires an overflow hole, why don't those sinks have one? We don't have kids, so I don't have to worry about them overflowing the sink. If the plumbing code absolutely requires an overflow hole, I believe there are overflow plugs I can purchase, but I'd rather just avoid the overflow hole to begin with. Does anyone happen to have access to the 2012 IPC?

I would suggest that you don't use bleach. It is going to breakdown the seal of the drain. I'm guessing that the stink is coming from the pipe and not the overflow drain.Maybe something was dropped down there that has sunk into the bottom of the dip in the pipe. Have you cleaned your drain? I clean mine with 1/4 cup baking sod, pour this into the drain, then follow by pouring 1/2-3/4 cup vinegar. Let it do its magic, 10-15 minutes or as long as you want. Then flush with very hot water. You will notice the water drains more easily. Woah la you have a clean drain. I do this to all of my drains, kitchen sink, bathroom sink, shower drain, and bathtub drain. If I had a dishwasher, I would do it too.