Brake Servo Booter for 1968 Volvo

I am replacing my brake servo booster with a new unit (a Laycock brake servo used on MGs). The brake booster is for single circuit brakes. I have dual circuit brakes and assume that the brake booster operates only on the front disc brakes. Is this correct??

which model is it? The 120's had that thpe of booster and only single circuit brakes . Later 120 and ALL other volvos have twin circuit with a booster bolted direct to the master cylinder, so your Laycock booster will only be any use on an earlier 120 with single circuit brakes .

booster 

I think you may be right about the booster only working the discs, something I read just last week mentioned this. BTW, I need to do the same mod on a single system so would like to follow your experiences and method through this thread.

Brake drums tend to have a self servo action, so a booster would probably make them lock up too easily. Much more suited to disk's and more progressive I'd imagine.