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Discussion Starter #1
So I was working uber and I was stopping at a stop sign all of the sudden I have no brakes just barley was able to stop. Upon looking at my driver side brake line my caliper line was loose I tightened and tried to bleed the brakes on that caliper and it won’t bleed now but also before this happened I hear a hissing sound from my brake ONLY when car is running and I realized it goes away if I remove brake booster vacuum line that goes to the engine. I checked the diaphragm and there is no cracks in the seal either.

~260kmiles
3.5L Motor

Things I’ve done
Changed master cylinder from used car
Swapped diaphragm anyway
Checked for leaks on brake lines and swapped 2 lines to the ABS unit which also ABS unit never worked.
bled brakes
changed diaphragm
smoke tested using booster vacuum hose
tested check valve
changed caliper driver side rebled

Could it be a completely failed brake booster? Or do I need a new ABS unit? Or you guys think its the vacuum hose? I’m pretty sure the vacuum hose no leaks I checked by holding one end and blowing in the other and I checked the check valve and its working properly .

Video: 20201014_132446
 

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Do you have a hard pedal when stepping on the brake pedal, or does the pedal go down to the floor? When you changed the master cylinder, did you bench bleed it first before installing it? I have had good luck in the past, on different vehicles, cracking open the bleed valve and letting gravity fill up the lines and calipers before doing the normal bleeding procedure.
 

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Discussion Starter #3
Do you have a hard pedal when stepping on the brake pedal, or does the pedal go down to the floor? When you changed the master cylinder, did you bench bleed it first before installing it? I have had good luck in the past, on different vehicles, cracking open the bleed valve and letting gravity fill up the lines and calipers before doing the normal bleeding procedure.
i dont have access to a bench so i had to bleed it with car im pretty sure its bled correctly i even performed a reverse bleed on it too both ways. my pedal goes all the way to the floor and i hear a hissing sound can be heard in the video posted above. I think its a vacuum leak but the vacuum lines dont seem to be leaking which leads me to believe its the booster.
 

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I've used those with good luck before. Normally, if the booster is bad, the pedal will be hard, without much braking action. The booster has a rubber bellows inside the steel tank. Vacuum from the engine causes a negative pressure on the bellows which makes it want to collapse when you push the brake pedal, giving you brake pedal assist. It fails do to a hole in the rubber bellows, or a loss of vacuum. Also it can fail due a leaking master cylinder seal, which fills the booster full of brake fluid, stopping the bellows from being able to collapse. Did the master cylinder bleeding kit fix it?
 
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