I purchased the above mentioned rebuild kit from fastlane - it included the thin o-rings. My old ones were still pliable so they went back in. I can't imagine a failure of a 'thick' o-ring would result in total loss of brake boost. I had 5-7 inches of vacuum with all three of the thin seals - still more vacuum than when one of the old valves disintegrated ( 0'" vacuum - the plastic remnants and tiny spring were sitting on top of the piston when I took it apart)

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tgingrich
'83 300TD 282k
'83 300D 239k
'82 300SD 204k
DFW