Those photos clearly show low-mounted master cylinders, different than I saw yesterday in the Kirkham document I looked at (
http://www.kirkhammotorsports.com/book_aoe/aoe_16.pdf) - perhaps that document relates to later production.
Regardless, given the low mount, there s/b a residual pressure valve somewhere in the rear brake line, unless there's one built in to the MC. Even if it's not the source of the current problem, it's worth knowing where it is.