A lot of times the rear master cyl on old bikes is bad and unrepairable and you can't get a new one. I have found a solution. I bought one for a Valkyrie off eBay and made it fit the 1100. This was done with the fuel tank out, but should not be a problem with it in place.
The Valk MC has bolt holes already the proper size to tap to 8x1.25 for the original bolts.
I then mounted it to the frame in the OE rear bolt hole and the upper hole in the MC with a spacer 3/8" long w/8mm hole.
I then made a bracket of 1/4 x 3/4 steel with 8mm holes 1" C to C to fit from the OE front hole to the lower hole in the MC.
Bracket mounted to MC with 8x1.25 allen head screw.
Mounted to frame with OE bolts, a nut is used on the back side of the bracket, could have tapped threads in it but didn't think of it in time.
2 washers were used to space the bracket out from the frame for alignment.
Had to work up a little slack in the brake line to get it to curve around to clear the swing arm. A new SS braided line would be a good thing.
The OE supply hose is too short. You will have to get a 3/8 hose compatible with brake fluid, 3/8 is a little tight on the OE reservoir and loose on the MC fitting but will work. Didn't even have to adjust the rod length for pedal height.