[mwebb]

so
you do not want to have hi pressure in a fuel tank .

newer systems vent to atmosphere through a charcoal canister which absorbs the HC as the pressure vents

when the engine runs
a valve opens that draws air through the canister and the absorbed HC gets burned in the engine
in closed loop so every thing stays at stoich the valve is duty cycled to control
PURGE FLOW ,

the system can close the EVAP VENT VALVE and open the PURGE VALVE to put a little vacuum on the tank , then with the FUEL TANK PRESSURE SENSOR monitor the rate of vacuum decay to see if the tank leaks
vacuum will decay faster in hotter temperatures as the fuel evaporates and the vapor pressure helps to decay the vacuum .

that is one common way to detect fuel tank leaks
there are other ways.
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what was the question ?