Brake Problem

[MarineHawk]

So, I’ve essentially got 72K of problem-free miles out of my 06 H2.

I was out of town for a few days. On Wednesday, my H2 drove fine. On return, I had my two little boys in the H2 to go to the park to kill some Orcs or something. On my slightly-sloped driveway, I put it into reverse shortly after starting up the vehicle. As I gently rolled back, as always, I put the brakes on to slow out of the driveway. The brake pedal felt entirely normal with pressure required to push it down, but literally nothing happened. It didn’t slow me down at all. By that, I mean that when I pressed all the way down on the brakes, it did not slow the vehicle down in the slightest. I got my boys out; sent them inside; and tried again a few times. Same result—forwards or backwards—the brake pedal had pressure, but did nothing.

I called Onstar, and they sent out a tow truck. The operator, tried it himself. He noticed that, in park, when he pushed down on the brakes, it made a screeching sound. In park. He immediately said something like: “It’s brake booster. It’s bad. It supplies power to the brake lines. In can tell from the sounds that that likely is what it is.” He towed it to the Cadillac dealership formerly known as the Hummer dealership where I bought it.

As it was well into the afternoon on Saturday, I knew it would be Monday before anything useful happened.

I talked to the service department this morning, I reported all of the above, and the guy told me that they test drove it twice and found nothing wrong. The brakes worked perfectly. Before he could say much, I told him that I was not going to drive this 8,000lb vehicle until they figured out what was wrong. I don’t want to be going 65 mph when the brakes completely stop working. He said that they would look at it again tomorrow, but can’t find anything wrong.

Does anyone know what might be wrong and what I should do? I don’t want to kill anyone with my unbrakable vehicle.

Thanks in advance for any advice.