Any time you connect something mounted on the engine and something mounted somewhere else, you need to use flexible lines because the engine is not stationary. That's why brake lines are hard (because they mount to the frame, which does not move), while the brake hoses are rubber (because the axle that the brakes are mounted to moves.) That's why the fuel line from the gas tank to the fuel pump is hard (because it's mounted to the frame which doesn't move), but the connections to the fuel pump are rubber (because it's mounted on the engine, which moves.)
