Brake Master Cylinder Problems of Chevrolet Sonic

Master cylinder and power break booster need to be replaced at 42800 miles.

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I was driving down the street (straight and flat, no turns or hills) at 25 mph and went to use my brakes. To my surprise there was no power assist. The car only has 48k miles on it and had never displayed braking problems prior to this incident. The car did not have issues at the stop sign just a block before. As it happened a few blocks from my house, I drove it home and parked it. Besides the lack of power assist, the motor started to idle rough-- it would surge to 2500 rpm and then slow down to 700 rpm, all with no throttle input. When I popped the hood, to look for an obvious loose hose, I did not see anything that looked out of the norm. However, there was quite a bit more noise than normal-- it was a combination of hissing and whirring. Not the normal sound the engine would make while running with the hood open. There were some noises coming from the brake booster area (driver's side by the firewall) that I had never heard prior to that day. The car has a 1. 8 four cylinder and a 5 speed manual. The issues did not impact the clutch pedal in any way, so I doubt it is the master cylinder. As I noted it seems to be a brake booster issue. . . However, I have not been able to get it to a garage yet as the issue just happened. (I am no mechanic, but have a decent understanding of how cars work. ) either way, it was quite a surprise. While I am a male in good physical shape, even I needed to stand on the brakes to get the car to slow down from only 25 mph-- had I been going faster, I would have likely needed to use the e-brake to slow the car. Again, at this time the car has not been repaired, but the issue happened just yesterday. For what it is worth, the temperature was cold at the time of the incident (15 f), but there was no snow or ice on the road, and it was sunny out.

See all problems of the 2012 Chevrolet Sonic 🔎.

I was driving on the highway at between 55 and 65 mph. The side I was on had a long, steady incline upward. I had set my cruise control to about 63 mph. As I was going uphill the cruise control was supplying fuel to get to and maintain my speed at 63 mph. I was approaching another car in my lane and decided rather than pass, I would slow down. They were going quite a bit slower than me so came up upon them quickly. I pressed the brake to slow down but the brake was hard as a rock. It would not budge so it did not disconnect the cruise control and my car kept accelerating. I panicked and tried to use both feet on the brake. It still would not budge. I was within 2 car lengths of the car in front of me when finally the car reached the speed I had set the cruise control for and the brakes finally depressed. In hindsight, I had options I. E. Putting the car in neutral, turning key off, turning to avoid a collision, shutting off cruise control, but this car was new to me, and in that moment I fully expected the brakes to work. Once I was in control, I tried to recreate the problem with no one around, knowing my options now. It happened repeatedly. A service tech also had it happen to them. First attempt at repair did not fix it. Second attempt did. It was a bad check valve. Apparently the check valve let air out of a booster, so I lost my brakes? the check valve also let fluid fill the booster. The master cylinder and vacuum were replaced as well. Looking back, there were signs, but no obvious ones. A random hi-pitched noise (air squeaking out the check valve). Loss of brake fluid. Dealer had to fill it when I bought it and I added some at 6 months. This was a dangerous and scary experience. I question the design to dis-engage cruise control and the quality of the check valves themselves.

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