mwerae

At night when the head lights are on and I step on the brakes the dash lights dim.

Why is that?

MetalEd

It sounds like a common problem.
Did you upgrade your rear/brake lights to a brighter bulbs or an updated lighting system (sequential tail lights) or even headlights to brighter ones? Are you running a lot of aftermarket electrical items (radios, amps, lights, GPS, etc.) You could be running at your charging systems limits and when those brake lights go on, you are draining the electrical. This can happen on some cars just by putting brighter headlamps in.
If this is the case, then the only options are not to run the devices, upgrade your charging system (alternator) or if it is not killing your battery or doing other damage, just live with it (but don't add or turn on anything else or you could end up stranded with a dead battery!)