If I hit someone’s garage while test driving their car, am I legally bond to repair it?

Your factual statement provides that you were driving someone else's vehicle when the brakes appeared to fail and as a result you caused damage to a garage door. You ask if you should pay to have it fixed. Under the law the ultimate responsibility would lie with you if the damage was due to your faulty driving or with the car owner if the damage was due to a faulty vehicle. However, most states have laws as they do in Kansas that provide that every vehicle should have insurance to cover the damage to someone else's property should an damage producing accident occur. In Kansas the damages described would be covered under the vehicle insurance policy and thus would be paid by the vehicle owner’s insurance. Considering the same an argument could be made that regardless of fault you should not have to pay for the damages since the same should be covered by insurance and if the vehicle was uninsured then the owner should bear the cost since he insisted that you drive the vehicle without informing you that the vehicle was uninsured.

The issue, legally, is fault: who is at fault, or responsible for the accident. If you hit the garage because you were driving carelessly, you would be liable; however, if the cause of the accident was defective brakes on the car, a car which was not yours or otherwise under your control (so you were not responsible for the brake maintenance), then you should not be liable for the damage.