Hi Nicholas, haviing reread my posting I would like to clarify that the early Sierras had a limiting valve rather than a proportioning valve - the difference being that the limiting valve only comes in to operation at a point where the rears will lock up. A proportioning valve works throughout the barking range, it is not allowed under IVA rules as it can be adjusted after the test and materially affect the brake balance. Some centres have allowed ones that have been welded up but I think they have stopped that now.
Best and all that,
Enoch
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