The sealed system that brings your vehicle to a stop includes the GMC brake lines and hoses connecting all the parts. If there is a leak of any kind or degree, the hydraulic fluid in this system will be replaced by air. Since air compresses and hydraulic fluid does not, the brake pedal will feel soft to the driver and more force will be necessary to be applied to bring your truck to a safe stop. The brake lines and hoses are what connect the force being applied in the master cylinder to the brake calipers and wheel cylinders located at the four wheels of the GMC vehicle. This system is composed of pre-formed metal lines as well as rubber hoses in order to connect to the calipers and/or wheel cylinders on your GMC truck. If there is a crack, puncture or break in a braking line or hose, a puddle of clear thin oil will appear below it on the pavement where the vehicle is parked. The only way to resolve this situation is the replacement of the broken GMC brake line or hose. To locate the place where the leak is occurring can be accomplished with an inspection of the brake line and hoses when there is a load on the braking system, or the brake pedal is being depressed. The leak will appear as a fountain type of spurting action until the force is not longer being applied in either the brake line or hose. This time frame is only a few seconds so look for the location of the puddle so you are in the right area before force is applied to the system on your GMC truck.