Hosam
Line 3 seems to be your fuel tank pressure line, so unplugging it will cause you to lose all pressure to the tank. Don't unplug it after you fill the tank with fuel, (THE DOT) it is there to release the pressure in the tank from the previous flight, so that you can refuel and be ready to fly again.The little filter thingy (before the fitting on the engine case) is called a "special high frequency check valve" this is part of the cline regulator assembly. This should be in the pressure line, oriented in the correct direction so as to trap pressure in the tank.(small brass end towards engine).