Since so much has been rebuilt, and you hear the air hiss, I'd tend to agree with your diagnosis.
It could also be an internal MC leak, but that wouldn't explain the air noise.
I've heard most rebuilders will call the booster not-rebuildable. However, others have found more skilled mechanics that have done a rebuild.
Booster is a hard part to find new, but is probably still available as a Ferrari MC/booster combo, but at a large cost.
Larry
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