There's different levels of service but I'll assume it's old and you want to refurbish it as best as possible. Basically you're going to have to tear it down to the motor. Thus, when it's all apart you can try to do the following:
1. Remove and soak separator in CLR or other hard water remover. Use gloves...and scrub hard. Rinse and dry.
2. Replace water pan gasket (could be a one or two piece foam gasket).
3. Clean Hepa filter if your unit has one.
4. Check spider (if the unit has one, this is under the separtor). These will and can crack. If it's damaged, replace it.
5. Continue to tear down to the motor and check motor seals, brushes, and main filter. Replace, clean, and/or adjust as required. Sorry I'm not more specific but it really depends on your model and how old and worn it is.
6. Replace main motor bearings. There are usually 2 to 3. This is a real pain...and may best be done by a local shop.
Eric K., November 2008
