Brisket Rub Question

I could be wrong, but don't believe that the rub is going to do anything to keep the meat from drying out. I believe that has more to do with the cooking technique than anything else. I like to foil my briskets around 160 to 165 internal until they're tender. I put some liquid in the foil to help the process and to add some flavor, usually some sauce thinned with apple juice. Then they rest in a dry cooler for 2-6 hrs. They always come out tender and almost always juicy. Brisket tends to dry out real quick after it's sliced, so I like to paint it with a little sauce too.

As far as the technique and timing,
Juicy and tough = undercooked
Tender and juicy = just right
Tender and dry = overcooked
Falling apart = way overcooked

Brisket can be tough to get right. If it isn't perfect the first time, make adjustments for the next time.

A good starting place for small quantity rubs is 1/3,1/3,1/3 by volume. Kosher salt, sugar, and flavor. Take from one add to the other according to your tastes. When using a dry rub (marinade) the salt will initially draw moisture to the surface. Best results will be had by allowing the now concentrated surface moisture to draw back into the meat over time. For relatively thin cuts an hour or two is beneficial whereas thicker cuts like brisket may take several hours and even thicker cuts like pork roasts may be improved by injecting . In this way you are basically "dry brining".

Rubs don't keep moisture in. They are strictly for flavor. Additional flavor and moisture can be created by injecting your brisket with the flavorful liquid of your choice. Beef Broth is a great place to start.

I like beef broth with onion soup mix