Is this under ambient pressure, or pressurized?
The rubber is just glued onto the bottom of the keg. If it's leaking anywhere other than a post/poppet/dip tube/lid, it means there's a crack and I'd say the only way to fix it is to weld it. To be honest, I wouldn't feel safe using a keg that can't even hold liquid at a modest pressure.
Where'd you buy the kegs?