Hey all
So long story short, i was doing some work on the JK and in order to gain clearance to remove something, had to unbolt and pull the master cylinder from the brake booster. After putting it all back together, i started hearing a hissing sound coming from behind my dash/instrument panel when driving. It is constant whether braking or not but it does change pitch when pressing the brakes however the brakes work fine from what i can tell.

I pulled the master cylinder back out and noticed that the O-Ring had not seated correctly so i reseated and bolted it back up. Still hearing the hissing sound. I checked the vaccum line and didnt notice any leak but went ahead and clamped it anywyas. Still hissing...

Any ideas? I am pretty sure it is a vaccum leak from somewhere. It would make sense that it is a leak between the MC and BB since it happened right after i removed and reinstalled it. The seal between the master cylinder and booster definitely doesnt look air tight but the o-ring looked fine when i reseated it. Could i maybe pull it and reinstall with some RTV gasket sealer or is that a no-no? I cant really hear any hissing from the engine compartment (likely because engine noise) but can hear it pretty clearly from inside the cabin.

Any input is appreciated.

 

no one has run into this?

 

I just noticed the same hissing sound. I was able to isolate it to the inside portion of the brake booster. I squeezed the rubber boot that is connected to the brake pedal, in doing so, I could eliminate the hissing. There must be a seal on the inside that is leaking. This just began when the weather turned very cold. I found some one else that stated it too started when the temp dropped.

I have not determined where the lapel is or what caused it.......more to follow. BTW, this is my third booster on my 2008 JK.