Abstract: The invention relates to a shuttle valve designed to retain fluid in a pressurized system when the cap or closure of the filler neck is damaged or left off. In a preferred application, the shuttle valve of the present invention is designed to be installed in the filler neck of an oil storage tank incorporated in a turbine engine of an airplane or other high speed vehicle. The valve head comprises a piston which is disposed to move slideably and is biased by gravity, or other means, to seat in normally closed position near the inner end of a cylindrical sleeve which is suspended coaxially from an annular flange in the mouth of the filler neck. The cylindrical sleeve has an axial length which substantially exceeds that of the piston, which in one embodiment is a hollow cylinder closed at one end.