so imma make it short and sweet and to the point, when i hit the brakes on my 240z i hear an unusual vacuum coming noise from inside the car, and also when the brakes are pressed my rpm's shoot up. could it be my brake booster?

Yes, sounds like the booster diaphragm may be bad....I think it could also be the check valve.

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.

More than likely, it's the brake booster. I had the same symptoms on my '71 and replaced the booster about a month ago.

Dennis

My '72's booster failed in January. Started as hard pedal feel followed by reduced breaking and engine RPM rev-ing upward and downward (stalling) when braking. The part lasted 37 years! It was due...

Edited November 18, 200916 yr by GreenZZZ

Thanks for the comments! I'll replace the old booster.

Thanks for the comments! I'll replace the old booster.

Any idea on where to get a new one, or at least a re-furbed booster. I have the same issue here.

Any idea on where to get a new one, or at least a re-furbed booster. I have the same issue here.

Nevermind, plenty of re-furbs available at auto parts dealers

On a related note, I guess the general concensus would be that if you have gotten any brake fluid in the booster, you need to replace it also? I probably drained a half cup out of it.

Found this: http://info.rockauto.com/Cardone/Detail3.htm?/53/535245-01.jpg

Is it the booster for the series 1? I have the booster still on my car so I'm not sure if this is the same.

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.

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.