1997 GMC Jimmy

When you start the car with your foot on the brake, does the pedal move down slightly? If not, check your booster operation. Is it does, do it a couple of times to verify that the hose isnt collapsing and causing no vacuum assist. If the pedal doesn't return normally, check the gasket under the master cylinder cap. If it is swollen or doesn't fit properly like it is too big, your brake fluid is contaminated and will require replacement of most of the rubber components of the brake system. Good luck

When I start the truck, the brake petal moves down quite a bit. Do you think maybe I just need to buy some brake fliud?

Thats good that the pedal is dropping. It should not go more than an inch or so. If it does, your back brakes are out of adjustment. Your front brakes are disc and should not require a great deal of effort to stop the vehicle. Definitely make sure the fluid in the master is to the top of the light colored plastic divider in the resevoir or to the bottom of the cap.