Clamp all 4x flexi brake pipes and pump the pedal is it hard? if not look at the master cylinder undo the bolts to servo and gently prize back any fluid at front of master?
If pedal is hard remove one flexi and try pedal it may show which circuit is your problem like a leaking wheel cylinder or brake caliper.
Had a sierra that was 12yrs old and had brake fade turned out been the master cylinder new one fitted spot on, but you do need to check the braking system before replacing just the master.
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