Q: air is seen going into the master cylinder,

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I have a 1999 Tahoe, when the break is applied there is hissing air pushed into the rear wheel side of the master cylinder, and you can see the bubbles. Is my problem the master cylinder or booster ?

The brake master cylinder should not be getting air into the back of the master cylinder piston seals. The brake booster is under vacuum and when it is released to push on the master cylinder it should vent out the pressure. I recommend replacing the brake booster and master cylinder to stop the problem.

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