expansion valve testing?

The bulb "tends to" close the valve as the suction line cools. This helps regulate flow back to the compressor. This is superior to an orifice/ capillary tube setup which has no regulation, therefore it better handles variable conditions, the worst of which EG is a got condenser (hot day) which causes more refrigerant flow into a cooled down "box" (passenger compartment) which let's say you are in a shaded area and lower heat load, would tend to flood refrigerant. The TXV better regulates this

Since this the bulb/ diaphragm is a relatively small, enclosed system, and has a moving part, it can fail, and it doesn't take much of a "molecular level" leak to allow the pressure in the bulb to drop