Dual Master Cylinder

These are very likely "proprietary" and they certainly are not "standard" fittings. "Standard" fittings for those lines typically take (I forget) either 7/16 or 1/2 wrench, and they are ALWAYS 3/16 tube

You don't have the original nuts? Parts stores used to sell bushings for those to attach "brake tees" back when trailer brake controllers were hydraulic