[Richard (EV8G)]

The bore of the original 39 MC is 1-1/16", or 1/16" larger than the dual MC. This will work, but will require slightly more pedal travel to push same amount of fluid, but pedal will be a little easier to push. You do need to confirm that the pushrod stroke (resulting from full available pedal travel, including about 3/8" for free travel at pedal and 1/16" at pushrod) is equal to or exceeds the full stroke of the MC. See my recent other post regarding dual MC's. Also, IF the new MC has no built-in residual pressure valves, will need to use external 10lb valves as near to the MC as possible. I am aware of ONE MC # that has built-in valves for 4-whl drum brakes, but there may be others; someone may know...

You don't say what year/model your vehicle is, but space above the dual MC
can be an issue/problem, as well as toward the rear of where the original MC
was located... Need access for the typically larger "lid" so have to cut a bigger
hole usually...