Certainly, when the bore in a conventional cast-iron master cylinder (or wheel cylinder) has become badly pitted and replacements are NLA, sleeving is necessary to achieve a good seal and prevent damage to the rubber cup seals which slide through the bore.
In the Treadle Vac, however, the rubber hydraulic seal never really moves. Instead, the plunger/piston slides through that fixed seal, pushing brake fluid out from the otherwise closed chamber to the wheel cylinders by simple displacement. It's when that plunger surface becomes pitted that the hydraulic seal could be damaged. As such, the BTV master cylinder body doesn't require sleeving in the conventional sense; replacing the plunger is the solution in that case.
However, I now realize why some BTVs might require sleeving. The underlying reason can be found in the 55-56 shop manual section on brakes, where you'll find a paragraph on inspection of the disassembled unit, advising the rebuilder to:
Quote:
Examine the hydraulic cylinder bore 1" from the open end. The surface must be free from scores, deep scratches, or corrosion, and be satisfactory for sealing with rubber hydraulic seal.
I can see where such issues with the die-cast master cylinder body in that area could cause leakage into the vacuum shell, and how they would likely be stopped by sleeving just that portion of the bore.
Now, none of the units that I've personally rebuilt or repaired with my own two hands exhibited such problems, but that was 20 years ago. With more of these cars being resurrected from barns and fields, perhaps that work is more necessary these days, but I doubt if it has anything to do with a failure shortly after a professional rebuild.
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