As to liability, here's a very long answer.
My business partner is an attorney with experience in product liability.
I wondered what liability we have driving 40 year old trucks in the first place so I gave him 4 scenarios representing a flawed replacement part, just plain accident, lack of maintenance and owner modifications.
Grab a cup of coffee and enjoy the read!
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So my question is: in the following scenarios; how fast will you get sued and loose your house when your brakes fail, because of a failed hose, and you run over and kill a small child carrying a puppy!
1. You replace a brake hose with an OEM spec one that has a manufactures defect.
2. You run over debris in the road and it cuts the brake hose.
3. You never bothered to replace the 40 year old hoses on your truck "because they looked pretty good".
4. You put together some adapters to make a brake hose you had in your garage work instead of buying an OEM one.
His response:
As a general proposition, the owner of a vehicle may be liable for damage it causes. Liability depends on whether the cause of the injury was due to neglect or intentional act of the owner. If the cause of loss was due to a modification of the vehicle, then everyone involved in the modification is exposed to liability. Generally, fault depends on a legal concept of lack of due care. If the modification was done with "due care", then no liability attaches even though the modification was the cause of loss. In practical terms, it is almost impossible to convince a jury that an owner modification to a vehcile was done with due care. Unless the owner is a professional mechanic, it is highly unlikely the owner will escape liability, regardless of how well the modification was done.
Therefore, given the high probablility of liability for losses caused by an owner's modification of his vehicle, I see the potential for laiblity in all four instances you proposed. In evey case, however, the brake hose must be the "proximate cause" of the injury. "Proximate cause" is a legal term that usually means there must be an identifiable causal connection between the injury and its cause. For example, an owner modified brake system would not be the proximate cause of injury from an exploding tire.
In each of your examples, liability attaches if the injured party can show a lack of due care that was the proximate cause of the injury.
Example 1: The question there is whether the owner did the replacement properly and performed the tests and checks required for the job, regardless of the defect in the hose. His work has to be equivalent to that of a professional mechanic. Both the owner and the hose supplier are exposed to liability, but the owner may be able to show that the "proximate cause" of the injury was the defective hose and not his work.
Example 2. If you can show lack of due care when you drove over the debris, there may be a causal connection between that event and the injury. The injured party can examine whether the owner replaced the hose in the correct position. "Unavoidable" is a very elastic term in accident litigation. However unavoidable the situation may appear, the injured party can test the circumstances to see if the driver could have, in the exercise of due care, avoided the debris. The party who left the debris on the road would have the same opportunity. You still have the issue of whether the owner's modification/repair was done with due care.
Example 3. What is the standard for replacing brake hoses? 20 years? 40 yeasr? Never? If in the exercise of due care, the owner should replace them at some point, then the owner may be liable, assuming a causal connection between the failure to replace and the injury.
Example 4. Adapting a hose to work on the vehicle is subject to examination for due care in the design of the replacement and the work done. Keep in mind that most auto manufacturers employ legions of engineers to do thousands of tests on equipment they design and build. The owner is likely to be judged in that category when the jury evaluates his design and work.
Determination of liability is a fact-intensive matter. Change just one fact slightly and the result might be completely different (who wins, who loses). The answer, of course, is INSURANCE. Insurance companeis write policies expressly for modified vehicles such as sleeper cars, race cars, off-road cars, etc. I think it would be a serious mistake to attempt to insure a modified vehicle without disclosing to the insurance company that it has been modified and, if required, the details of the modification. Insurance companies can decline to cover your loss if they conclude that you did not fully explain to them what risk they were insuring.