[72kool]

Mine is not 4x4 but the brake setup looks the same. A hard brake pedal is usually 1: drum brakes are out of adjustment and need loosened. or 2: an issue with the booster. these are vacuum boosters that run off of vacuum through the manifold/carb. I would check to make sure the hose going between these is solid and not cracked, make sure the plastic inlet on the booster is in good shape. if you know the booster is good, i would check that you are getting adequate vacuum from the engine to the booster at those speeds. how does the engine run? did you have anything done to it recently? I know that when i had my carb rebuilt, there was 2 gaskets in the kit and used the wrong one first. the difference was the vacuum port was different and it didnt allow adequate vacuum to the engine and brake booster.