One of the thermal sensors is for the normal brewing temperature and the second one is for steam temperature, and is activated by the steam switch. Both sensors turn on the green light, and turn off the heating element, when the appropriate temperature is reached.
The one way spring valve has nothing to do with the operating temperature or pressure. It is only there to keep the water from leaking out of the boiler until the switch for espresso, which starts the pump, is actuated.
The wiring diagram is found here: http://www.partsguru.com/user/Wire%20Dia...
The tear down instructions are very clear and easy to follow, and the photos are excellent. However, there is a basic misunderstanding, as to how the espresso machine functions. Steam has nothing to do with pressure needed to press the hot water through the espresso grounds. The pump not only draws up water to the boiler, it also develops 15 bar pressure, pumping the hot water through the filter. Steam is produced, for the purpose of steaming milk, by activating the steam switch.