98 sunfire GT brake problems

It seems when using the brakes at lower speeds they work properly.... But when I am on the highway they don't seem to respond as well- I have no clue?! Any help/suggestions would be greatly appreciated :)

When you say they don't respond well what do you mean by this? It is possible that the vacuum booster on the car is going bad. During high speed operation the vacuum can leak off and cause a lack of boost. Check the one way valve on the master cylinder. It is easy to find and inexpensive to replace. This may cure the entire problem. But get back with a better description of the lack of response.