If you have drum breaks on your trailer they are electric, so no master cylinder. I believe 1977 and 1978 were the only years before 2000 that came with hydraulic disc breaks and a master cylinder from Airstream, note that was vacuum over hydraulic. Some newer trailers (I think around 2007 and special orders) came with electric over hydraulic disc breaks.
Of course that doesn't mean they weren't added aftermarket.
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