2004 Chev Trailblazer has air leak noise when brake pedal is depressed [engine running] may be power brake booster? I am beginning to loose presure when applying brakes, brake pedal presure is fine when engine not running, fluids are fine,. Easy fix?

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A brake booster assists the driver by applying additional pressure to the brakes for an easier stop. Therefore, if the booster is going bad usually braking gets more difficult.
This could be a sign you are in need of a brake inspection. If you feel the pedal is losing pressure it's possibly air in the braking system. A bleed and flush of the fluid would be recommended.
Keep in mind the braking system is a sealed system, low brake pad linings will cause the fluid to drop a bit as well.
Inspect the brakes, replace if necessary, flush and bleed the system as well.
If the symptom continues you can move to the next step.
Until then you need to know if the braking system is in good condition and working normally.
tim@10w40.com
-Tim West