well first step in almost any brake diagnosis is to take the wheel off and look at the brakes, look for anything broken and then try bleeding them also check the big hose coming out of your brake booster (right behind the master cylinder) to make sure it isnt broken you may be loosing vacuum from the engine because its not at a higher RPM

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1990 Ranger Extended Cab
2.9L V6 5 Speed Transmission
Planning to put a 347 and a T-5 in it eventually