Bill,
+1 on what Dave said.
Also if your brisket or pork butt finishes to early you can set smoker to 140 degrees, remove the butt or brisket and double wrap in foil, leave the door open briefly to drop the temperature. Put the butt or brisket back in the smoker and you should be able to hold it 4-5 hours using this method.
I do this all the time and it works great.
Greg