I was chilling in the parking lot waiting for my brother to get off work and I had my keys turned to where only my radio was on. I was tapping my foot on the brake and I noticed the voltage went down. So I watched my volt meter and if I pushed in the brake my volts went from 12.8 to 12.7 and when I let off the brake it would go back up to 12.8 volts. Is this normal? Why is this happening?

It just hit me that it might be my brake lights comming on but remember the key was turned to "radio".

 

your brake lights come on whether the key is in the car or not.
and why are you watching a voltmeter?

 

you had you key on auxilary which is acuttaly intended to turn on not just your radio but gives power to any thin that might be wired in that circuit also if you press on you brakes the lights will come on even if you don't have your key in the ignition

 

I have a digital meter that i can turn on and off with a switch and since I had nothing else to do i was fittling around with things in my car and I happened to notice this. And thanks, i didn't know that the brakes came on even if the car was off. I mean, i have never been at the back of my car while i was pushing the brake in. Thnx.

 

Yes, this is normal...if you have an older car with brake lights bulbs. You are turning on a circuit in the car. ...

 

With the voltmeter resolution you have voltage could have gone from 12.751 (rounded up to 12.8v) to 12.749 volts (rounded down to 12.7v) So do not be too alarmed. Try a better quality voltmeter with more resolution.
A .1 volt drop does seem like a alot for just some break lights!
Say you have 50 watts of break lights, that would be about 4 amps of current (watts=voltsxcurrent so amps=50/12=4) . Using ohms law we get a battery internal resistance of 25 milliohms.(V=IxR so R=V/I or.1v/4amps=0.025ohms) That is high for a battery. But maybe your battery is not very good anymore?

 

even the new cars's brake lights go on when you tap the brakes with the car off....

just a few taps (with the engine off) and it should goo stiff but still the light will turn on for the lightest tap...