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How to test faulty leaky brake booster?

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After months of scratching my head as to why I have +13% A/F when low G/S MAF, smoking my motor countless amounts of times, I THINK my brake booster is to blame. ( the auto-mated Current MPG on the dash goes to almost 0MPG when I am applying brake to a stop.)


I did a pretty hard brake at 50mph to catch a right-hand turn, and my brakes felt very weird. braking power felt like it was pulling left and right. pedal stayed firm.


So.. how to test??


Can I blow into the black hose coming out of the brake booster, cap it with my thumb, and see if it holds pressure? seems logical..


just wondering if there are any tips or tricks in this matter


thanks alot



p.s is swapping a new one in hard? or do i have to deal with bleeding brakes (which with that, i will bring it into a shop)

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I was suspecting the same issues, and found a few ways to test.


First if you do a standard boost leak test, is anything leaking? If no you’re good up to the check valve.


From there you have a few options. Unplug the booster and plug the hose, do your fuel trims get better? Using a hand pump to leakdown test the booster, does it hold vaccum?


Or the simplist tell: got hit the break petal after it sitting over night (without motor running). Does the booster still boost your inputs? If yes, it still has vaccum and this no leak.

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