The secondary piston is corroded and must be replaced in the master cylinder for a 69 with disk front brakes. The master cylinder is US stamped 309 casting. Does anyone know where to get a new secondary piston? None of the overhaul kits contain the secondary piston. From the shop manual it mentions the correct piston has 3 ribs. I did find one for a corvette but the original part numbers are different. The car is numbers matching and I need to keep the original US cylinder.

Thanks