It installs in the rear line ONLY.
Please read:
http://auto.howstuffworks.com/master-brake2.htm
Many people choose to use only the proportioning valve and not the metering valve for several reasons, including the fact you can buy it seperately and adjust it to suit your needs.... and 4 wheel discs normally dont need front metering. Front metering can be quite useful for disc/drum in the snow.