Sounds like you've got it all covered, but other things i'd check are the backplates - make sure there's no movement and also the lines - you haven't mentioned replacing them, so if they're the original they may be degrading internally and creating 'valves' with small flaps of rubber.
"Y mae dafad ddu ym mhob praidd"
1972 4/4, 1981 MGBGT, 1984 FLHT1340, 1980 XLH1000, 1990 ZX10
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