Just out of curiosity....
Would it be possible to use a dual circuit master cylinder in a single circuit system if you plugged one of the circuits?

I'm with Joel on this one. And to go to all that effort it will be just as easy to plumb the brakes right. As a stop-gap, I think you would be better off just keeping the old single circuit system until you could do it right. As mentioned, you can do a f/r split and get just as effective a system as the diagonal one, just not as redundant.

Mike!

I'll gladly trade you a new single circuit mc for your working dual mc.(vertical mounting holes)

The point would be-
If one planned to upgrade to a dual circiut system, had a dual master cylinder but not the other bits yet and wanted to do part of the job and still be able to use the car in the meantime.
Hypothetically...

Man, I make this change on my amazon some time ago. Now, I´ve two brake circuits, one front, one rear. I use the brakes parts from another amazon for complete the parts. My brake cylinder is a standard horizontal holes bracket, witha a round metalic "donna" adapter to attach it to the original holes (on the past, Ipd made it, available from time to time on ebay) - the repro brake cyl (with vertical holes) is too high priced for my taste. You can see my setup on http://www.volvoadventures.com/121JoaquinPeru1000.jpg and http://www.volvoadventures.com/121JoaquinPeru1009.jpg.

Happy volving

Joaquin
Rojo 121 Amazon
Lima Peru

That makes sense. I thought of doing this as well, have a used dual master but need a rebuild kit.
I didn't understand the "plugging one circuit" concept. Still don't. Others may give their opinion as to whether one circuit would do the entire trick.

My plan was to split the dual - fronts and rears and leave it at that. The complete dual diagonal setup requires new 4 pot calipers, and different backing plates for the fronts, as well as the additional plumbling (new lines to to fronts and the rears).
Crille at Veteranproduktor in SC has the mc rebuild kit, but I have yet to determine that the one I have is rebuildable. He can probably get you a MC as well.
If you can't find a serviceable dual MC with the vertical mounting holes, I hear tell that you can get or make an adaptor plate for the later (1800 and 140) MC which has horizontal mounting holes. I see these (horizontal mount mc) on ebay now and again, but rarely the one that's a direct bolt-in to a 122.
Let us know your progress.

I am not sure how master cylinders works. But I would be afraid that the blocked one would stop and pedal movenent so you would get little brakes. If you tee the two ports into the one line for now it should work
Joel