I thought I'd add some info, since I just did this and was reading through some threads regarding brake upgrades and this one needs some straight facts...

The 123 and 126 brake master cylinders are the same. Same part number. That's why you can use the "stock" 123 MC. The only difference with the 126 cars is that the ones that have ABS have a different MC, which is neither here nor there for this upgrade.

Also be aware that you'll probably have to do something with the vacuum lines. The too-long 123 main line works, but it does move the check valve/splitter farther away from the other lines and those become too short. So either shorten the check valve to brake booster section by about 2" or make the smaller lines longer. I plan on making a custom main line and then doing some cleaner/more reliable work to the vacuum system. Good opportunity to do that.

I also see no way you can do this without removing the master cylinder completely from the car, so plan on doing a flush, along with any other brake work at the same time.

-Rog