Go back to basics,check brake hoses,caliper piston seal leaks and sticking pistons.Bleed brakes with front of car elevated in correct sequence.Check wheel bearings,can cause brake pad knock off.I have done three Subaru 4 pot caliper conversions 94 Wrx,94 Legacy Rs and Legacy 2002 Gt and one Brembo conversion a 2007 Legacy Bl.None of these cars required a master cylinder upgrade.