1) Sanding the "thermo" is not enough - lightly sanding the mating surface (usually steel) is necessary.
2) Thermostat is NC (Normally Closed), NOT Normally Open (NO). It opens (and thereby cuts off the current to the heating element) when the thermostat temperature (~194 degrees F) is reached (the thermostat should be reading the temperature of the steel floor that it is contacting, which should be the same temperature as the water in the percolator, since both water and steel are both better thermal conductors than air). If the thermostat is not intimately contacting the mating surface, it will not be reading the temperature of the mating surface and, instead, will be reading the temperature of the "air" in the enclosed bottom section of the percolator, which has been heated by the heating element. The air in the bottom (enclosed) area of the percolator gets hot very quickly. The water and the steel floor take much longer to heat up, so if the thermostat is not in intimate contact with the steel floor of the pot, it will read the air temperature instead of the steel temperature and will shut down (Open up) before the water reaches 194 degrees.
3) Most thermostats have some type of device to press the thermostat against the steel floor for tight, intimate contact. Could be a spring or piece of silicon rubber, etc., but it is critical that the pressure device be aligned to hold the thermostat flush and tight against the steel floor (need intimate thermal conductivity between the thermostat and the metal floor).
4) The spring and the tube are important for several reasons:
a) the spring holds the bottom of the tube in intimate contact (tight) with the "well" in the bottom of the pot, so that the water that is heated in the "well" (superheated water) all goes up the tube (does not escape out the sides of the bottom of the tube to the bottom of the pot) and percolates through the coffee, heating the water from the top down,
b) the tube must be free of obstructions or restrictions, so that all of the superheated water in the well can freely go up the tube, continuously, removing some heat from the bottom of the pot and transferring it to the top of the pot. Over time, the temperature of the water at the top of the pot will closely match the temperature at the bottom of the pot.
c) the water going up the tube heats the pot of water (percolates) "from the top down", instead of just heating the water from the bottom up, the hot water going up the tube slows down the heating up of the water near the bottom of the pot, extends the percolating process, and extends the time before the thermostat actually "senses" the floor/thermostat to be at 194 degrees.
d) If you removed the tube and just plugged in the pot of cold water, even with a good thermostat in intimate contact with the floor, you would have a situation where the floor of the pot was very hot, but the water was not, and the thermostat would trip (turn off) before the entire pot of water got to 194 degrees, because the metal floor could get to 194 degrees before the whole pot of water got to 194 degrees (steel is a much better thermal conductor than water). I would not be surprised if the pot of water was only at 150 degrees when the thermostat tripped at 194 degrees.
The principle is simple and effective. There are a few things that can go wrong, and the thermostat is one of the most important things, but not the only thing that can contribute to poor percolating.
Easy Maintenance Items, to be regularly addressed:
A) The inside of the well must be very clean, so that the maximum amount of heat can be transferred to the inside of the well, to "superheat" the water in the well, allowing it to go up the tube.
B) The Tube must be clear of obstructions, and
C) The spring on the tube must be strong enough to apply good pressure on the tube against the well.
D) The bottom of the tube should not be damaged and must fit tightly against the well.
More Difficult Maintenance Items, but should only need to be addressed occasionally, if done properly:
E) The thermostat must be in intimate contact with the metal floor, perfectly flat (flush), with some pressure against the floor, and with surface contaminants (oxides) cleaned off the surface of the floor and off the surface of the thermostat that contacts the floor.
Last piece of information: Once the thermostat trips Open (shuts off the main current to the heating element), there is normally some type of "warming" circuit that kicks in, maintaining the coffee/water temperature, usually at a temperature lower than the "trip" temperature of 194 degrees....but this is not always the case. I have tested coffee pots where the "warming" circuit keeps the coffee/water at higher than the thermostat temperature (above 194 degrees), in the 200+ degree range...and the thermostat does not care, because it has already tripped OFF.
Remember that the percolator does NOT have a timing circuit to turn off the percolator after a fixed amount of time, as many of the new Drip Coffeemakers do, which is typically two hours. You need to unplug the percolator to turn it off.