Slow brake pedal return on a vehicle with a hydraulic brake booster may be caused by which of the following?

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When a brake pedal returns slowly with a hydraulic brake booster, the speed of the pedal return depends on parts inside the booster that push things back to the resting position and release hydraulic pressure. The booster uses a power piston and a spool valve, each with springs that help reset them after braking. If the power piston return spring is broken, there isn’t enough force to push the piston back promptly, so the pedal lags. If the spool valve return spring is broken, the valve may not reset properly on pedal release, keeping hydraulic pressure in play and slowing the pedal’s return. Internal binding—due to contamination, wear, or sticking components inside the booster—causes friction or sticking, which also resists movement and slows pedal return. Any of these conditions can produce slow pedal return, so all of the above are plausible causes.