judgment on the verdict

In October 1958, plaintiff Chester Vandermark bought a new Ford automobile from defendant Lorimer Diesel Engine Company, doing business as Maywood Bell Ford, an authorized Ford dealer. About six weeks later, while driving on the San Bernardino Freeway, he lost control of the car. It went off the highway to the right and collided with a light post. Vandermark and his sister, plaintiff Mary Tresham, suffered serious injuries. Vandermark had driven the car approximately 1,500 miles before the accident. He used it primarily in town but drove it on two occasions from his home in Huntington Park to Joshua Tree in San Bernardino County. He testified that the car operated normally before the accident except once when he was driving home from Joshua Tree. He was in the left-hand westbound lane of the San Bernardino Freeway when traffic ahead slowed. He applied the brakes and the car started to make a little dive to the right. It continued on across the two lanes of traffic till she hit the shoulder. Whatever it was then let go and he was able to pull her back into the road. He drove home without further difficulty. He took the car to Maywood Bell for the regular 1,000-mile new car servicing before using it again. He described the freeway incident to Maywood Bell’s service attendant. Maywood Bell’s records do not indicate that any complaint was made. After the car was serviced, Vandermark drove it in town on short trips totaling approximately 300 miles. He and his sister then set out on another trip to Joshua Tree. While driving in the right-hand lane of the freeway at about 45 to 50 miles per hour, the car started to make a little shimmy or weave and started pulling to the right. Vandermark tried to pull back but could not. He applied his brakes gently to see if he could straighten her up but could not pull her back to the left. He let off on the brakes and she continued to the right. He tried again to put on the brakes and she would not come back. All of a sudden this pole was in front of him and they smashed into it. Plaintiff Tresham testified to a substantially similar version of the accident. A witness for plaintiffs who was driving about 200 feet behind them testified that plaintiffs’ car was in the right-hand lane when he saw its taillights come on. The car started to swerve and finally skidded into the light post. An investigating officer testified that there were skid marks leading from the highway to the car. Plaintiffs called an expert on the operation of hydraulic automobile brakes. In answer to hypothetical questions based on evidence in the record and his own knowledge of the braking system of the car, the expert testified as to the cause of the accident. It was his opinion that the brakes applied themselves owing to a failure of the piston in the master cylinder to retract far enough when the brake pedal was released. Hydraulic fluid should have been able to escape into a reservoir above the master cylinder through the bypass port. The expert also testified that the failure of the piston to retract sufficiently to uncover the bypass port could have been caused by dirt in the master cylinder, a defective or wrong-sized part, distortion of the firewall, or improper assembly or adjustment. The trial court struck the testimony of the possible causes of the failure of the piston to retract on the ground that there was no direct evidence that any one or more of the causes existed. Plaintiffs brought this action for damages against Maywood Bell Ford and Ford Motor Company. They pleaded causes of action for breach of warranty and negligence. The trial court granted Ford’s motion for a nonsuit on all causes of action. The trial court directed a verdict in favor of Maywood Bell on the warranty causes of action. The jury returned a verdict for Maywood Bell on the negligence causes of action. Plaintiffs appeal.