Disc brakes rear. Will crown vic master cyl work?

Not sure what year CV you are referring to, but it would probably be better to use a MC from a 4 wheel disc '75 or '76 Thunderbird(it fits your original booster's bolt pattern-bolt-on application-no mods other than brake line fitting size differences) and use an aftermarket disc/disc proportioning valve(you will still need to change fitting sizes but the aftermarket unit uses double flare-the I believe CV prop valve uses a metric/bubble flare). When I went 4 wheel disc, I also installed hydroboost at the same time and used an '80 Corvette disc/disc MC due to fender side exit ports and matched it with an aftermarket disc/disc proportioning valve. Hope this helps