[T2] Can disc master cylinder be used with drum brakes?

[Warren McKean]

Gents:
I have a 1970 Baywindow (bought new). I plan to upgrade to front disc
brakes soon, but have an immediate need for a master brake cylinder. Can I
use the brake cylinder that will be necessary for the front discs with my
existing front/rear drums? Also, since most folks install discs in the
front only, does the 1971 brake cylinder have a retention valve in the rear
hydraulic line?
Warren McKean
1970 Camper