Muf--
A couple of thoughts for you.
1. Have you replaced the check valve assembly? Maybe something bad is getting into your boosters.
2. The boosters contain rubber; rubber ages, and if you are buying old break boosters--it is no wonder that they are failing.
3. Somebody wise once told me don't rebuild the brake booster or the master cylinder. Replace them when they fail, with new. I think your experience is the reason why they said this.
By the time you add up all the used boosters, and a rebuilt cost, you might have exceeded ONE new one which might last for many years.