Abstract: PCT No. PCT/SE78/00065 Sec. 371 Date June 28, 1979 Sec. 102(e) Date June 25, 1979 PCT Filed Oct. 26, 1978 PCT Pub. No. WO79/00241 PCT Pub. Date May 3, 1979A fluid tank arrangement of the kind adapted to deliver a fluid to a consumption unit and comprising a reserve fluid container (16), which is operable to deliver the reserve fluid into the tank (3) when the fluid in the tank has been consumed, said container being arranged in association with a filling hole (4) in the tank. According to the invention, it is proposed that said container is combined with a cap (1) for the filling hole in the tank so as to compulsorily follow the cap when it is removed from and replaced onto the tank and that the container, when containing said reserve fluid, is adapted to retain the reserve fluid independently of the position of the cap and tank. The container is preferably an absorption element.