New DOT regulations as of July 1st state that you must take at least a half hour break BEFORE you hit 8 hours. This is from the time of punch in. It has to do with driver fatigue. The break should not be taken in the first hour or the last hour of your shift. Our managers went over this last week with us. I'm not sure about the vehicle out of service situation though.
This is the new rule, federal- so I think it covers all of us. It is an HOS if you don't get at least a half before you work 8. (We are all DOT, right?)News Release - Federal Motor Carrier Safety Administration
Since every district seems to make up their own rules about break and driving on break, I will just add that about 3 or 4 years ago we were told we have 8 minutes to split up before and after break. So, it could be 4 mins after last scan to 4 minutes to first scan after break or however you want to break it up.