When changing to discs, you need a proportioning valve, along with the master cyclinder..
Preferably from the same make of car you got the discs from, or from a 4 disc version of your car (sorry, this applies to hondas, but dunno about your make, I think they call it with the same name). What it does is split the pressure to the front and the rear brake calipers to provide uniform braking.
It works something like a front/rear brake bias controller, except it's non adjustable.
If you use the stock proportioning valve from your car that has drums, your rear brakes may actually activate first, causing rear end skidding and reducing braking performance overall, since a drum needs a different amount of pressure to provide the same amount of braking. And brake modulation will probably be an issue as well.