As said a MC used for discs must have no RPV under the brass seat where the brake line screws into the drum half (iirc they went from them to a small round disc in the wheel cyls behind the cups to maintain a slight psi) so the odds are less but with what is going on here I would for sure check. pull (unscrew) the 3/16 line from the drum port & stick a drill bit down in the brass cone & if it has one it is just a piece of neoprene and a spring so you will easily feel it if it has one with the springiness & if by chance it does we will post how to pull it & that is assuming the MC is the correct bore (I ain't sayin it ain't) but there might be a mismatch of parts (I hope not, you have alot invested in this), but just going from the prior posts, I ain't up on this conversion.