A valve assembly comprising: (a) a main valve body comprising an outer valve body and an inner valve body and having a longitudinal bore, wherein: a.1 the outer valve body has an inner section and an outer section, said inner section of the outer valve body being adapted for mounting in an opening through a tank wall such that said outer section of the outer valve body will be external to the tank wall; a.2 the outer section of the outer valve body has an outer end (11,11A) adapted for connection to a source of vacuum; and a.3 the bore of the main valve body defines an inner valve seat in the inner valve body and an outer valve seat in the inner section of the outer valve body; (b) a piston longitudinally movable within the bore of the main valve body and having an inner end, a central section and an outer end, said piston defining: b.1 an inner sealing surface adapted for engagement with the inner valve seat; b.2 an outer sealing surface adapted for sealing engagement with the outer valve seat, with the outer sealing surface being larger than the inner sealing surface; and b.3 at least one longitudinal flow path; (c) a first magnet incorporated into the inner end of the piston; and (d) a second magnet retained by a magnet holder incorporated into the inner valve body, with the first and second magnets being oriented to produce an attractive magnetic force between the first and second magnets, and with said magnet holder being configured to define at least one longitudinal fluid flow path; such that: (e) the attractive force between the first and second magnets biases the piston toward a closed position with the inner end of the piston in sealing engagement with the inner valve seat, such that fluid flow through the valve assembly is prevented; and (f) a vacuum of sufficient strength applied to the outer end of the outer valve body will urge the piston out of sealing engagement with the inner valve seat toward an open position in which fluid can flow outward through the valve via the longitudinal flow paths of the piston and the magnet holder, with the attractive force between the first and second magnets retaining a residual value effective to bias the piston toward the closed position when the vacuum strength drops. (a) a main valve body comprising an outer valve body and an inner valve body and having a longitudinal bore, wherein: a.1 the outer valve body has an inner section and an outer section, said inner section of the outer valve body being adapted for mounting in an opening through a tank wall such that said outer section of the outer valve body will be external to the tank wall; a.2 the outer section of the outer valve body has an outer end (11,11A) adapted for connection to a source of vacuum; and a.3 the bore of the main valve body defines an inner valve seat in the inner valve body and an outer valve seat in the inner section of the outer valve body; a.1 the outer valve body has an inner section and an outer section, said inner section of the outer valve body being adapted for mounting in an opening through a tank wall such that said outer section of the outer valve body will be external to the tank wall; a.2 the outer section of the outer valve body has an outer end (11,11A) adapted for connection to a source of vacuum; and a.3 the bore of the main valve body defines an inner valve seat in the inner valve body and an outer valve seat in the inner section of the outer valve body; (b) a piston longitudinally movable within the bore of the main valve body and having an inner end, a central section and an outer end, said piston defining: b.1 an inner sealing surface adapted for engagement with the inner valve seat; b.2 an outer sealing surface adapted for sealing engagement with the outer valve seat, with the outer sealing surface being larger than the inner sealing surface; and b.3 at least one longitudinal flow path; b.1 an inner sealing surface adapted for engagement with the inner valve seat; b.2 an outer sealing surface adapted for sealing engagement with the outer valve seat, with the outer sealing surface being larger than the inner sealing surface; and b.3 at least one longitudinal flow path; (c) a first magnet incorporated into the inner end of the piston; and (d) a second magnet retained by a magnet holder incorporated into the inner valve body, with the first and second magnets being oriented to produce an attractive magnetic force between the first and second magnets, and with said magnet holder being configured to define at least one longitudinal fluid flow path; such that: (e) the attractive force between the first and second magnets biases the piston toward a closed position with the inner end of the piston in sealing engagement with the inner valve seat, such that fluid flow through the valve assembly is prevented; and (f) a vacuum of sufficient strength applied to the outer end of the outer valve body will urge the piston out of sealing engagement with the inner valve seat toward an open position in which fluid can flow outward through the valve via the longitudinal flow paths of the piston and the magnet holder, with the attractive force between the first and second magnets retaining a residual value effective to bias the piston toward the closed position when the vacuum strength drops.