Check valves include a valve seat and a movable valve member, or piston. Movement of the piston is what allows fluid to either flow or be prevented from flowing. A magnetic check valve is provided that contains an annular piston, which is configured such that the fluid flows through a central aperture in the piston. Fuel cell systems include a cathode loop which is in contact with the proton exchange membrane (PEM). The PEM is sensitive to fowling by contaminants that may be present in the ambient air. The magnetic check valve apparatus disclosed herein prevents backflow of air through the exhaust side of the cathode loop. The valve is used on the inlet side of the cathode as well.