Where is the brake booster on a 1986 CJ7?

On a 1986 Jeep CJ7, the brake booster is located on the firewall, typically behind and to the left of the master cylinder. It's a relatively large, cylindrical unit connected to the master cylinder by a pushrod. You'll likely need to look closely and possibly use a flashlight to see it clearly as it might be partially obscured by other components.