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brake booster?


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  • 9 months later...

I have almost the same problem with my 240Z -70 (manual gearbox), but the hissing sound is there only when the brakepedal is not pressed. When I press the brakepedal the sound stops. Any ideas what causes this?

EDIT: I have no problem with the engine rpm's.

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  • 2 years later...

I am just about to change out my booster as I am having the same symptoms (hiss when brakes are applied with car running and RPM's rise by about 300). I'm curious why the RPM's rise with this condition. My best guess is that the carbs are being leaned out somehow, when air is seeping past the booster diaphragm. Anyone else care to offer thoughts.

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Yep, the hiss you hear is most likely a vacuum leak coming from the broken diaphragm causing extra air to be introduced into the intake. When mine failed my idle became unstable. Because yours went up you may be running a little rich and the extra air from the leak is bringing you closer to stoichiometry.

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