PSU88

I just fired up my engine for the first time since putting in a 383, and I replaced the master cylinder and power booster with a chrome unit. After bleeding the system and checking for leaks, everything seemed ok prior to start-up (brake pedal was solid after pumping it a few times while the car was off).

So, after starting it today, there is a hissing noise coming from inside the firewall by the brake pedal (under the dash) only when I press on the brake. When I lift my foot from the brake, the hissing noise goes away. Also, the pedal goes to the floor way too easy after starting up.

It was too late tonight to get under there to see if I cound "see" where this was coming from. Has anyone run into this problem before? Any thoughts would be appreciated! It's a 78 vette.
Bill

Peterbuilt

Bad Booster, someone said new=not tested. PG.

gingerbreadman1977

mine has been doing that for 5 years if its the same sound as yours. i dont always notice it to be honest but it sounds like a bicycle pump pumping air when you pump the pedal. is that what yours sounds like? i can only hear mine when at idle and press the brake firmly.if im driving i dont hear it. once the brake is applied the noise goes, its only there during the action of pressing the pedal. my brakes are excellent and have never given me any trouble so im not touching anything .

GunPowder

Have the same sound (bicycle pump sound gives a good idea of how it is like) with a brand new booster (could be bad nevertheless).

PSU88

Yes, it definately sounds like a bicyle pump hiss, but it continues the whole time I have pressure applied to the brake and doesn't stop. When I remove pressure from the brake and the pedal rests on the striker, it stops.

I do have cruise control, and I just replaced the brake vacuum switch...perhaps it has something to do with that? I'll have to try to isolate where the sound is coming from, which is kind of hard with engine running, applying brake and looking up under the dash

Bill

mtcraigco

Your car may explode at any time,

I think you should ship it to me and I'll test it for awhile for free!



J/K

gingerbreadman1977

im hearing you 88 about getting under there and looking for the noise while u have to press pedal at same time. that doesnt sound easy. you might have to take the seat out on that side and have someone else sit up back with a broom handle doing the brakes while your under there. doesnt sound correct if the noise is there constantly while braking.

PSU88

Thanks 1977...wasn't fun taking the old booster out out or putting the new one in...my back still hasn't forgiven me.

I was talking with a friend about vacuum leak, and we're thinking it may be coming from the brake vacuum switch...at least that's what I'm hoping. I'll start by plugging the cruise control vacuum and see if I still hear the noise when applying the brake. I had to fashion a 5 port vacuum fitting for the cruise control and may have messed something up there....

brooksman9

Just ran into this issue on my car I am restoring. All I have read points to a bad booster. Have you resolved your problem yet? Thanks

joes86vette

Yes - This happened to my 86 Roadster. It was in fact a Bad Brake Booster. Probably your most likely place to start looking.

Good Luck!

Cheers ~ Joe

brooksman9

Thanks Joe. Thats what I was afraid of. I was just hoping it was something I could fix that would have been less expensive.

Gale Banks 80'

Go to Home depot and Buy some cheap hose. Hook one end to Your Vacume Lines and the other end to your buddys Minvan parked outside your garage. Now you can hear all the Vacume leaks. If that dosn't find it Hook it to the Brake Booster and listen again.

brooksman9

LOL...How did you know my buddy has a minivan?

PSU88

Brooksman - As it turned out, the hiss was coming from my cruise control switch that mounts above the brake light switch and not the booster. I replaced all of the cruise control components when I had the engine out, including new hoses. I had to fabricate a vacuum hose connector for 4 hoses since I couldn't locate an aftermarket alternative. I think that had something to do with my problem. When I applied the brake, the cruise control switch would open, and vacuum would escape through that switch...I hosed something up on the vacuum line routing for cruise control.

So, for now, I just pulled the vacuum line feed off the manifold that was feeding cruise control and plugged it - hiss gone (but no cruise control )

Not sure if you have cruise control, but that was my problem.

Good luck!

Bill

brooksman9

Thanks Bill. My same problem is on my 77 Camaro that doesn't have cruise. Haven't looked back into it yet since I am about to pull the motor back out for a little more work. I'll deffinitely check all the vac hoses first though. Thanks again

...Roger...

Bill the hose going to the cruise control brake switch shouldn't have vacuum on it until the cruise control is engaged. The vac switch on the brake pedal is an emergency switch to dump vacuum from the actuator if the cruise transducer malfunctions. Sounds like you have some hoses hooked up wrong or a leaking transducer.
(This is from memory so I will go and look at a vac diagram and make sure ,so I reserve the right to blame old age if this is wrong. lol )