Q: How to testing and replacing the power brake booster on the Nissan Xterra?A: Depress the brake pedal several times with the engine off. Start the engine and turn it off after one or two minutes. Depress the pedal slowly several times. Apply 20in-Hg of vacuum to the brake booster, push in on the output rod, measure the length and adjust if necessary. To replace the power brake booster, remove the master cylinder, disconnect the vacuum hose, remove the clevis pin, remove the four nuts, slide the booster out and measure the distance between the booster and the hole in the clevis.