If the master cylinder goes bad it leaks brake fluid on the seal in booster, then the booster goes south and quits working, been there, you will need to change them both. Look for brake fluid running down underneath the master cylinder, if so they both are bad. Depending on the unit it's quite expensive to buy a rebuilt booster, if you can even find one. My 1 1/2 ton truck brake booster cost almost $1000 and that was years ago.
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