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the booster sucking fluid from the resivior. that is not possible. it could suck fluid from the piston bore, but two seals would have to fail to do so
You're correct in there are two main seals, but one is leather and almost worthless for retaining fluid. The other referred to as the cup seal is a single lip rubber seal where the inner circumference lip rubs the piston and the outer lip circumference does the bore. That piece does the actual work of sealing fluid. The ring seal is a third piece but not much more than decoration to keep fluid from falling on the ground as near as I can tell.
It may not be a terribly common occurrence but I have seen more than one BTV that has sat unused for long periods where due to whatever reason, the shaft has pitted in the area where the lip has sat. Suspect contamination in the fluid as the culprit but even if the damaged area is not in contact with the seal at all times, when the unit is put into service again, just in passing the rough area (even with a new seal) there is eventual damage and a leak--old hard 50 year old seals are another issue. Granted, new SS pistons are available, but not all go to the effort when rebuilding and just replace the seals that come in the kits. As to the new MC's, I and many others would appreciate info.
As to the different designs between manufacturers--that is another area where lots would like more info. If you could enlighten that would also be appreciated. That has been one area of speculation as to why Packards seem to have failures and other mfgs apparently don't--something different in the design.
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