One Brakelight is drawing a bit more current then the other one. This difference is causing the Sensor to go off. Sometimes by tapping the brake twice, you will get a small arch that may even out the current causing the sensor to 'be happy'. Let's try 'simple' first.

The Bulb sockets for the Brakelights ( lower corners of the Light assemblies) loosen both sockets a fraction of a turn.
The reason is this, Even with new circuit boards, the plastic housing under the Electrical traces for the brakes disappear. Meaning there is no plastic under the little fingers of teh Bulb socket. By loosening the sockets a little bit, you get a better contact to the Circuit board.

Also, make sure the white connector (wire harness) is snapped under the little black clip if it isn't, the connector can shake a bit causing the same unbalanced current draw. I'm playing with the same issue now. I have hard wired an entire set of Sockets to eliminate the circuit boards all together but I am too stubborn to just put them in.
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'75 Jeep CJ5 345Hp ChevyPwrd, two motorcycles, '85 Pickup: The '89 Volvo is the newest vehicle I own. it wasn't Volvos safety , it was Longevity that sold me http://home.no.net/ebrox/Tony's%20cars.htm