You can think of the master cylinder/slave cylinder relationship as a lever, with the lever ratio being the ratio of the areas. The smaller the area, the larger the pressure, but you have to make that up with a long stroke, just like in a long lever arm.
The Bendix Treadle-Vac has a 0.652" (some say 0.655") bore diameter, with a very long stroke, from what I can measure around 3". This is what allowed it to work with the small diameter vacuum booster, and be (marginally) acceptable in the case one lost vacuum.
I tried a Ford Courier dual master cylinder with it's 7" booster. It has a 3/4" bore size. The 7" booster barely fit down there, but all in all the installation looked great. I put some pictures up somewhere on another thread, I need to find it.
However, the pressure ratio is proportional to the square of the bore diameter (area=pi D^2/4) that means it has a pressure ratio of (0.655/0.75)^2 which comes out to about 75% of the original brake pressure.
It was NOT ENOUGH! I drove it and I had to push HARD on the pedal, and my neighbor was of the opinion it was not safe.
I have recently evaluated using the Wildwood TM1 externally mounted dual master cylinder. It has a 0.625" bore, but only a 1.3" stroke, so I don't think it would be acceptable, though I have thought about it.
Wilwood TM1 Master Cylinder, 0.625" boreThis Post was from: https://packardinfo.com/xoops/html/modules/newbb/viewtopic.php?post_id=233066