You have enough good helpful answers but I would like to add my silver dollar to the table.
This is my mixture I settled on finally after many briskets. It will get you to South Texas Brisket standards every time.
1 and 1/2 cups Salt
1/2 cup Black Pepper (20-23 mesh black pepper)
1 Tablespoon Cayenne
If you use kosher, you will get a shell like crust that is kind of flaky and hard.
I recommend table salt which is what most use in Luling, Lockhart, Taylor, Elgin, and here in Bulverde at my home etc.
If you use more black pepper, you will get the Opah's Pepper Coated Beef Jerky (Fredericksburg) taste effect which is what I call what you experienced already.
I do hot and fast on my cooks which is something else you should be doing for this kind of rub to work (see Donnie aka Funk et al)
Let us know what happens brethero.