Brake pedal causes; tach dip, temp gauge dip

[BurbankJalopy]

Hello
When I press the brake pedal my tach dips about 500 RPM (engine doesn't actually slow down, just tach reading), and the temp gauge dips too. I've cleaned the ground pods, cleaned the coil mounting connection, added a ground strap from the engine to the chassis, and even added a ground from the battery neg terminal to the negative on the drivers side rear taillight post. I've also jiggled the wires around for bad connections/breaks and they all seem ok. I've even checked the piston return springs. Anyone have any ideas?

[manoa matt]

Does it happen all the time, or only when the running lights are on? I'd check the wiring at the brake light switch, which is mounted behind the brake pedal arm.