When I step on the brake in my 88 SE, the headlights and panel lights dim a bit. On releasing the brake, they come back up to full strength. The voltmeter dips a volt when this happens. The rest of the car is fine, no other electrical problems of any kind. Normal for these cars?

 

No, not normal.

What voltage is the car idleing at?

It should be around 13.5 to 14.4. You may want to read it with a digital multi-meter to make sure as the factory gauge is only <.5 accurate.

If it is below that the alt is starting to fail or you have bad grounds to the engine or battery.

If the charging circuit (read alternator) is operating correctly then there should not be any sustaned dip using any accessory or brake lights.

 

I'd say alternator also. I think All cars have a system to regulate the voltage. As I understand it when you draw power from the circuit by introducing something else like break lights, turning on the heater, or stereo etc, the system balances the draw out. Same with load. turn off the heater stereo etc, and more electricityis loaded and an adjustment is needed. You should not notice the adjustment made by this system.