The Sierra automatic brake adjusters, need the shoes to be fully "off" so the ratchet engages properly to adjust the mechanism to remove play as the shoes wear. An incorrectly adjusted handbrake cable (too tight) will cause this so I am sure that RPVs will do the same. I believe that you would only need to use RPVs if the master cylinder resevoir is below the brake cylinders as this could cause fluid to bleed back into the master cylinder. However I have a classic car (drum / drum) where the master cylinder is bolted to the chassis below the floor, so well below the wheel cylinders! There isn't a problem with bleed back because the master cylinder is fed from a remote reservoir up under the bonnet, so there is always a head of fluid above the brake cylinders. Peter.
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