The fluid used in the sensor is isopropyl alchohol. I was informed many years ago that they can be topped up using a syringe... but you need to know exactly how much to use as they are not filled completely, needless to say, I avoided supplying them.
The alchohol forms a mobile conductive path across a small ceramic potentiometer carbon track such as that found in a servo. As the sensor is tilted, the fluid moves around the carbon track changing the resistance. This results in a voltage variation within the circuitry and is converted in to digital pulses which vary in width accordingly. These pulses are the same as those delivered to your servos, so naturally when a servo is connected and the circuit board angle is altered, the servo moves in accordance with it.
