Your problem is two fold: using Foil and using a Cooler. have you ever herd the term Carry over cooking??? Look you have fallen into the trap of it works in comps so it must be the right thing to do crowd. If you want the to brisket to stay moist Stop doing what you are doing.
Pull your meat off the pit sit it on the counter and tent it loosely park a thermo in the center of the Cut and leave it alone. If it will be several hrs( 3+) before you eat let the temp drop to 160 then wrap it and place it in a cooler and place a folded bath towel on top. If your going to be eating with in 2 hrs leave it tented until the IT drops into the 150's before you slice it against the Grain, AND NEVER cut more than you need for the meal. If you slice the whole thing up it WILL DRY OUT because there is more surface area. Ever been to a Fancy Buffet where all meat is carved by a guy in a white coat and a funny hat that aint done to be different it is done to give o the best they can offer Moist, juicy and full of flavor.
I never met a Cow that I didn't like with a little Salt & Pepper! Certified PORK-A-HOLIC