I was driving my wife's HHR today and after about 50 mile/80 km highway drive, I pushed the brake to slow as I was exiting the highway, and the pedal was rock hard. 5 minutes earlier there was no issue when I changed highways and did the exact same thing. I quickly realized it was the power assist not working. Once I arrived and stopped, I could hear a clicking under the radio area, as I pushed the pedal. I put it in park, unloaded what I needed to and when I got ready to leave, the brakes were working normal again. I drove all the way back home cautiously with no more issues and brake functioning correctly.

It was 19ºF or -7ºC, and sunny today.

Has anyone have any ideas what may have caused this and does the HHR use a booster for power assisted brakes or ...... ?

 

One possibility. Moisture in the vacuum hose could have frozen the check valve at the booster. This would keep the engine from being able to pull a vacuum from the brake booster.
Pull the rubber brake booster hose off and check it for moisture. Also maybe blow it clear with some compressed air.

 

The clicking is the brake-shifter interlock solenoid. You should hear it every time you step on the pedal when the car is in park.

You may need to have your brakes inspected for wear, sometimes worn out pads will do that kind of thing.

 

I believe this is going to be the issue, I parked it in my shop, because it is heated, and I'll being heading out to blow the vacuum line for the booster out.

Thanks Donbrew, the front brakes are new, about 3 months old. I think I'll double check to be sure the pads aren't binding in the sliders too.