Yes, look for the brake arm linkage. It is the rod right below the one leading to your throttel. Its the bottom rod on the throttel servo. If you look past the servo towards the front of the truck there should be a piece of fuel tubing with a plastic stopper holding it on. You can tighten the plastic stopper to tighten the brakes, and loosen it to loosen the brakes. Becareful though, if you over tighten it you brakes could always be slightly on causing break-drag. This is not good, always make sure the truck can roll free at idle.
Although that could improve your brakes, if you want your truck to stand on its head you have two options. 1) Get a stronger throttel servo, like an HS-625MG or a HS-645MG. 2) Upgrade your brakes with HPI's Dual Steel Disk brakes. Those brakes will give you insane stopping power and cost around 15-17$.