John Muir collapsed in 1980 or 81 . Problem was, by the time they sorted out who owned what, the market had collapsed (it did it a lot back then. you know, metals, oil up big and......hey) and the clients coulldn't sell. They got crushed.
They make you whole, but in time. Recent one was MJK Clearing. Largest SIPC failure to this date. Khosoghi and Deutsche Bank did them in. Maybe you can find out the procedure by researching that one.