By on September 11, 2018

General Motors has recalled over 210,000 late-model sedans and crossovers in the U.S. and over 19,000 north of the border after discovering the potential for a braking issue. The automaker blames the issue on rear brake calipers supplied by ZF, which can also be found on vehicles built by Volkswagen, BMW, and Audi.

It all comes down to trapped hydrogen gas in the body of the brake piston, which, when released into the brake fluid, makes for a mushy left pedal and reduced rear brake performance.

GM claims it was first notified of the issue by dealers after certain vehicles failed pre-delivery inspections. On June 14th, days after the automaker opened an investigation into the issue, ZF, which had already notified the German brands about the potential problem, alerted GM. The supplier had failed to properly chrome and temper the pistons, with the potential for small amounts of hydrogen to remain inside the piston bodies.

The automaker claims all of the gas would be released into the brake system within 23 days of assembly, but drivers would notice it after 15 days.

Included in the recall are 2018 and 2019 model year vehicles, among them the Chevrolet Cruze, Equinox, Malibu, Impala, Volt, and Bolt, as well as the GMC Terrain, Cadillac XTS, and Buick LaCrosse.

The obvious solution is for a technician to bleed the brake system and release the gas. GM claims it isn’t aware of any crashes resulting from the issue, and that 306 of the 375 warranty claims it received by the 23rd of August were for vehicles that hadn’t yet been turned over to owners. All of the faulty pistons were installed before July 17, 2018, it added.

[Source:, via Automotive News] [Image: General Motors]

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20 Comments on “Let It Bleed: General Motors Recall Targets ZF Brakes...”

  • avatar

    If the chrome plate and heat treatment was done wrong no amount of bleeding is going to fix that. That’s ZF’s fault, but isn’t it GM’s responsibility to bleed the system after assembly?

    • 0 avatar

      The system is bled after assembly, the gas leaks out over a 23 day period after assembly. From the story above…

      “…The automaker claims all of the gas would be released into the brake system within 23 days of assembly, but drivers would notice it within 15 days…”

      • 0 avatar

        Never tried to bleed hydrogen, (why is it present?), can’t it be bled like air?

        • 0 avatar

          Molecular hydrogen is incredibly small and it can worm its way into the crystal structure of many metals. It doesn’t technically react as much as it just sticks to various points in the structure (“physisorption”, though it can react and cause embrittlement and cracking in various metals and alloys).

          This is considered an equilibrium process so as long as there is enough H2 around, you’ll have a certain percentage of that gas moving onto/into the metal and a certain percentage leaving the metal. Once you remove the H2 around the metal, that same percentage keeps leaving until it’s all gone.

          You can speed this up through heat, vacuum and/or other processes either individually or in combination with each other. It sounds like these parts weren’t properly degassed at the factory.

          This process of physisorption is one avenue of research for storing H2 at low pressures (read: don’t need a heavy pressure vessel or cryogenics to store it safely).

          Source: I have a BSc in chemistry and did undergrad research in the area of metal/organic frameworks with an eye on how to store and release molecules in those frameworks.

          • 0 avatar

            @ notapreppie – hydrogen embrittlement was the first thing that popped into my mind when reading the story. It just seems to me left unattended it would cause the pistons to fail in some manner but I guess if it completely out-gasses in a month probably not a big deal.

          • 0 avatar


            Hydrogen embrittlement really only happens with odd metals like titanium and vanadium because they form hydrides. Hydrogen damaging steel isnt really embrittlement so much as it causes cracks. The H2 reacts with the carbon in the steel to form methane at really high pressures. Enough carbon and enough H2 and, we’ll, you can guess.

    • 0 avatar

      Let’s hear it for German engineering.

    • 0 avatar

      Right, so you bleed it and get good pedal for now, but the improper chrome plate will likely lead to premature corrosion and either a leaky or stuck piston down the road.

      • 0 avatar

        As long as the brake fluid is serviced at regular intervals, you shouldn’t experience any issues. Most caliper pistons are not chrome plated. This is just an extra corrosion protection measure.

    • 0 avatar

      This fix seems half-baked (sorry) to me as well. From

      Why do most airframe manufacturers require that parts that have been plated be baked?

      During the plating process, hydrogen is produced at the surface of the part being plated. This hydrogen has an ability to enter the structure of the material being plated and causing discontinuities of stresses. baking the part after plating relieves this hydrogen and any stresses which may have been introduced during the plating operation.

      There’s some more interesting reading at:

      It sounds like it could be a serious problem. I wouldn’t say a brake piston body is necessarily a highly-stressed part, but it is certainly a critical part. So why not replace the parts now and be done with it?

    • 0 avatar

      Is there some sort of chemical reaction between the brake fluid and the improper chrome plating that is leading to the production of hydrogen?

      How could there be any significant amount of gas stored in the caliper, if it was bled correctly to begin with?

  • avatar
    el scotto

    How did the hydrogen gas get into the brake lines to begin with? If the calipers left the factory with hydrogen gas in them; ZF’s fault. If assembly of the calipers, brake lines, and master cylinder somehow introduced hydrogen gas to the system; GM’s fault. All’s well that ends well; a Trifecta brake bleed will fix everything.

    • 0 avatar

      It’s a byproduct of the plating process. H2 gas is produced on the surface and the majority of it bubbles away. However, individual molecules of H2 can diffuse into/onto the metal crystal structure and will then slowly diffuse back out again.

  • avatar

    Why German companies do not recall their cars? As far as I know German cars undergo much tougher TUV periodical technical inspection tests than so called “emission test” in US. These cars simply will not pass the test.

  • avatar

    Tell me again why the ZF parts are better than the cheap Chinese parts?

  • avatar

    Swap the pistons for ones that are not plated, or ones that have been properly plated and baked, to boil off the hydrogen.

  • avatar

    Wouldn’t the answer be to merely open the brake reservoir cap and let the gas escape. Hydrogen is lighter than air…

  • avatar

    @Ion88: Trust me, if you’d ever bled your own brakes you’d know that’s not an option.

  • avatar

    2018 Cruze Hatch diesel here and it doesn’t seem to have this issue. Probably why it’s not under the recall.

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