Brake Booster- general signal failure

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"brake"light C0021-02 Brake booster general signal failure. Only on the hwy with no braking for 3mins. I had the P14 recall done last January ( they didn't replace the booster only added the housing to prevent water from getting in). When I turn the car off and then on it goes away. City driving I don't get the light. I did the 3 test that machinic's do to test the booster bump and they all pass. The only one that is questionable is the test of the check valve. The petal starts to slight rise after 20 seconds, but not fully.