Yes the master cylinder can leak into your booster and it's very common. As a matter of fact, booster that have traces of brake fluid in them can not be taken back for a core charge. If your booster is vacuum opearted, the leaking brake fluid into the booster will be sucked into your intake and the car will smoke heavy white fumes. It happened to me twice and will happen again if I continue to buy JUNK products from Autozone!!!!!!

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Meza
1993 190E 2.6
Advanced Diesel Systems Test and Research Engineer