I have a 1977 Airstream motor home based on a Chevy P 30 chassis with all four wheel disc barkes. I need a master cylinder gasket. Its not the same gasket or master cylinder typically used on a rear drum set up, Its much larger. the gasket has Bendix 2228633 stamped on it. It measures 6 inches long by 4 inches wide. Very large master cylinder fluid tanks. I can not find it at any big box auto parts stores. Does anyone here have one or a source ?