Dirty Water drops behind shampooer

You are over sudsing your recovery tank. The machine is made to shut off the air flow if water starts to be pulled into the motor, but bubbles want trigger that. So if there are a lot of bubbles in your recovery tank, the get sucked into the motor (Which will destroy it eventually) and then the moisture gets exhausted onto the floor from the motor.

Solutions
1. Use less soap (Read the directions, you may be using too much)
2. Use a different kind of soap (The concentration might not be right for your machine)
3. Use a defoamer (Rug Doctor has a defoamer)