brakes slowly go down ,and becomes hard to push . on 1998 GMC 1500 Pickup

when driving the brakes go smoothly, within mins, they become hard to push,and we end up in a jerking motion to stop the car.

Asked by for the 1998 GMC 1500 Pickup
Not sure about the specifics of the braking system on your GMC but a brake pedal that seems to become "harder" to press, more pedal effort required to stop the vehicle sounds like a brake booster that is failing.