Calipers require more volume than wheel cylinders. That is why ALL OEM have specific MC's for a disc/drum vehicle. Having a larger reservoir to account for pad wear is only one part of it and if you are religious about checking the level it isn't important, the volume pushed out of the MC is, which a drum/drum MC cannot match. Not to mention that if you do not remove the residual pressure valve from the front brake outlet you will have abnormal pad wear when using an all drum M/C in a disc application. M/C are cheap enough not to change it out to the correct setup.