so I have noticed that my 91 sedan's brake pedal doesn't really seem to be responsive after driving for a little while on the highway. If I press the brake pedal down for the first time (while on the highway) it will go down further than it normally does. Once I let the brake pedal go and depress it again then the brake pedal travels less and the brakes apply sooner. It doesn't happen all the time just sporadically but usually after prolonged driving without using the brakes. Stopping is otherwise still very good.

Is this a sign of a master cylinder failure or is my brake booster dying?

Thanks