The defective vehicle owner's liability depends upon whether the vehicle owner failed to use the care of a reasonable person to prevent the accident, which is up to a jury to decide on a case by case basis.
If the jury finds that you reasonably believed that you didn't have a problem with the car that needed repair and could foreseeably result in this kind of accident as a result of your investigation of the issue by talking to a mechanic, they you have no liability, although the mechanic might have liability to the person whose car was damages if the mechanic's mistake was one that a reasonable mechanic wouldn't have made.
If the jury finds that the vehicle owner exercised reasonable care, and that the mechanic, while mistaken, reached the same conclusion that a reasonable mechanic would have reached under the circumstances because the cause of the brake failure was uniquely difficult to diagnose correctly for some reason, then neither the vehicle owner or the mechanic has liability to the person whose vehicle was damaged.
In most civil law countries of continental Europe, unlike most common law countries which use a negligence theory of liability, the vehicle owner would have tort liability for the amount of the damage to the vehicle that was damaged, because in those jurisdictions, the definition of fault is closer to a strict liability definition than the definition of negligence at common law.