Well, technically they can be "rebuilt" by either using a good used bellows, or rubber cement or silicone on the bladder rip, or new bellows rubber if you have a source, but why rebuild when replacements aren't that expensive? Unless you just like tinkering and taking stuff back apart, just replace it and be done with it (if I had a good cheap source for vacuum bellow rubbers, I might change my mind; yes, they can be fixed, but it is very temporary...).

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