What is the test for establishing that defective brakes caused an accident?

[15] [16] The City contends that the evidence cited above is not of sufficient substantiality to support the jury's finding that defective brakes caused the accident. To establish causation, a plaintiff must prove that the defendant's conduct was a substantial factor in bringing about his or her harm. ( Padilla v. Rodas (2008) 160 Cal.App.4th 742, 752, 73 Cal.Rptr.3d 114 ( Padilla ); Williams v. Wraxall (1995) 33 Cal.App.4th 120, 132, 39 Cal.Rptr.2d 658.) Stated differently, evidence of causation must rise to the level of a reasonable probability based upon competent testimony. [Citations.] A possible cause only becomes probable when, in the absence of other reasonable causal explanations, it becomes more likely than not that the injury was a result of its action. [Citation.] The defendant's conduct is not the cause in fact of harm where the evidence indicates that there is less than a probability, i.e., a 5050 possibility or a mere chance, that the harm would have ensued. ( Williams v. Wraxall, supra, at p. 133, 39 Cal.Rptr.2d 658, italics added.)