Your problematic check valve is made of Alloy 20 (UNS N08020, 20% Cr, 29% Ni, 3% Cu, 2% Mo) and this is the "classic" austenitic alloy for dilute sulfuric acid service. In more recent times, Alloy 20 has been replaced with Alloy 20Cb-3 (UNS N08020, 20% Cr, 35% Ni, 3% Cu, 2% Mo), which has better resistance.

Confusingly, these alloys have the same UNS number. For a step up in resistance you could try a nickel based alloy like Hastelloy C-276 (UNS N10276) or Alloy 31(UNS N08031). In vacuum evaporation of viscose spinning bath solutions, Alloy 200 (UNS N02200) has reportedly been successfully used apparently because of the absence of dissolved oxygen and/or the influence of inorganic sulfates (sodium and zinc) (See MTI publication MS-1: Materials Selection for Sulfuric Acid).