Q: master cylinder or brake booster

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hello, I need to know if I need to replace my master cylinder or power booster. when car is off I get a little hard pedal. when I turn it on the pedal goes to the ground. I have no brakes at all. I replaced the calibers, rotors, etc. and bled the brakes multiple times. I get good pressure from the front brakes when I bleed them and little pressure from the rear. which I am told is normal. I have no vacuum leek in hose coming from the booster. when I tried driving a short way ( I know not recommended with no brake ) I found that when I pumped the brakes real fast about 6 times I was able to stop. I have no brake line leaks either I checked that. when I disconnected the check valve in the vacuum hose I was not able to stop at all. my engine does not stall or even change in idle like someone mentioned in one of your post. If it was the master cylinder that was bad, I would think I would still have some brakes? your opinion would be greatly appreciated

If the pedal is low, there is air in the brake lines or the master cylinder has an internal leak. If your vehicle is equipped with anti-lock braking, you have to either bleed the ABS pump and valving (per Factory Service Manual instructions) or use a bi-directional scan tool to remotely actuate the ABS system during brake bleeding. Otherwise, in an ABS equipped vehicle air will remain trapped in that segment thus causing a low brake pedal. If you do not have an ABS system, the only explanation for what you are describing is trapped air, an internal leak within the master cylinder (of course look for external leaks, but you have done that already), or possibly bulging rubber brake lines. If you have a firm pedal and yet inadequate brake pressure at a wheel cylinder, that would be due to a proportioning valve fault and/or a line blockage. If you desire the brakes be bled by a certified Mechanic, dispatched by YourMechanic right to your location, please request a brake system inspection/diagnostic and the responding certified mechanic will get this resolved for you. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you

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