When building my SS i put the stock savage servo in, after all its only a throttle / brake right?? The dual brakes would not stop the truck, and if they got wet from grass or what ever there was no brakes. No adjustments would help. After a rebuild i decided to use the old steering servo as the throttle brake. First run, touched the brake and the truck flipped over, bent one of the cooling fins on the engine head. I backed the adjustment scew away back but I could not get the truck to stop flipping over.
Eventually I took the bit of piping out from the linkage and shortened it, now the servo has more to travel before engaging.

If I slam on the brakes full the truck will flip, or at least raise the backwheels in the air. Cool, but bad for the diffs. Brake control is now as important as throttle control.
Morale of the story is.. change the servo. Use the steering servo for throttle / brake, and buy a decent servo for the steering.