in a fuel vapor processing system , a float valve and a check valve are provided in upper parts of a fuel tank to conduct fuel vapor to a canister via a fuel vapor passage . the check valve includes a low set - pressure valve and high set - pressure valve , and operates in two stages so that the internal pressure of the fuel tank can be favorably controlled by the opening of the low set - pressure valve when the fuel tank is being filled to full so that the sensor associated with the fill nozzle can successfully detect a tank full state , and any excessive rise in the internal pressure of the fuel tank typically caused by a high temperature of the surrounding environment can be avoided by the opening of the high set - pressure valve .