DON'T use a drum/drum MC with disc brakes. The (rear) reservoir for the front disc brakes will not be large enough for the front calipers. Use a disc/drum master (Rock Auto has them listed- 1972 DODGE DART 5.6L 340cid V8 Master Cylinder | RockAuto ) and remove the residual valve from the (front) reservoir for the rear brakes; just run a sheet metal screw into the brass seat and pop it out- the residual valve is right behind it, then reinstall the seat. The (front) drum reservoir is plenty big for 99% of the rear disc conversion kits out there, rear discs have a relatively small caliper volume since they do comparatively little braking.
The drum/drum part that you're probably thinking of is to use the drum distribution/proportioning valve since the bias remains fairly accurate for a four wheel disc system- more so than the disc/drum unit; although with the Pirate Jack rear disc system you'll probably need to run a separate proportioning valve in your rear brake line anyhow.
Are you talking about the Centric 13063015 , which looks remarkably like the original one I had ??DON'T use a drum/drum MC with disc brakes. The (rear) reservoir for the front disc brakes will not be large enough for the front calipers. Use a disc/drum master (Rock Auto has them listed- 1972 DODGE DART 5.6L 340cid V8 Master Cylinder | RockAuto ) and remove the residual valve from the (front) reservoir for the rear brakes; just run a sheet metal screw into the brass seat and pop it out- the residual valve is right behind it, then reinstall the seat. The (front) drum reservoir is plenty big for 99% of the rear disc conversion kits out there, rear discs have a relatively small caliper volume since they do comparatively little braking.
The drum/drum part that you're probably thinking of is to use the drum distribution/proportioning valve since the bias remains fairly accurate for a four wheel disc system- more so than the disc/drum unit; although with the Pirate Jack rear disc system you'll probably need to run a separate proportioning valve in your rear brake line anyhow.