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These units don't have a drain plug. The likely reason the brine tank is full is because the float valve in the tank which controls the water level in the tank has failed or the float is hanging up. The float could be hanging up due to a bad valve mechanism or some obstruction (but this is rare). You can either rebuild or replace the valve
Float valve failure is just coincidental. I don't see any mechanism where the bypass failure could have caused a float valve failure.
Or the other way around
No, the bypass is always under pressure and nothing in the softener mechanism can affect it.
Nothing in the softener mechanism can affect the bypass. They are entirely separate and independent. The fast drain mode failure could be due to an internal softener valve failure which may require a softener valve rebuild so if the unit functions under normal circumstances I'd let the fast drain thing slide given what it would take to fix it. The bypass is basically just a cheap plastic valve. Any strain on it (possibly from the piping or an impact to the piping) could cause it to crack.
You have to siphon or pump it out.
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