Probe Tender> sit a stick of butter on the counter overnight in the morning take what ever it is you use to check your meat with hold it over top of the butter close your eyes and slowly push it in to the butter do this 3-4 times. if you meat don't feel like that it anit done.
Flats can be tricky but I find that I get the best results at 275 F with them try 4 hrs then wrap in Butcher paper and cook to probe tender. Allow it to rest on the counter top wrapped until the IT drops to 150. The long slow rest with the gradual drop in temp is key to a tender & moist end product.
Adding stock to a wrap or injecting a water based solution don't add moisture to the finished brisket. As the meat cooks it draws up and wrings out all the water as it cooks like a sponge( this is why a cooked cut weighs much less than a raw one) it has to do this before the collagen can melt and convert to gelatin this is what makes it tender & moist. It is for this reason alone that cooking to probe tender is critical to a successful cook as no two briskets will ever cook the same because of the amount of connective tissues vary from Cow to Cow.
I'm a Proxy Vegetarian> Cows eat grass & I eat cows.