Vacuum Hiss Under Dash When Applying Brake?
Thread Starter
Intermediate
Joined: Jan 2009
Posts: 31
Likes: 0
From: Southlake Texas
Vacuum Hiss Under Dash When Applying Brake?
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
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
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
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 .
Thread Starter
Intermediate
Joined: Jan 2009
Posts: 31
Likes: 0
From: Southlake Texas
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
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
Drifting
Joined: Apr 2006
Posts: 1,911
Likes: 2
From: gold coast queensland
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
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
Thread Starter
Intermediate
Joined: Jan 2009
Posts: 31
Likes: 0
From: Southlake Texas
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.
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....
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
5 MOST and 5 LEAST Popular Corvette Model Years in History!
Joe Kucinski
2027 Corvette Buyer's Guide: Everything You Need to Know!
Joe Kucinski
10 Things C8 Corvette Owners Hate (But Won't Tell You)
Joe Kucinski
10 Best Corvettes Coming to Barrett-Jackson Palm Beach 2026!
Brett Foote
Every Corvette Grand Sport Explained! (C2, C4, C6, C7, & C8)
Joe Kucinski
Grand Sport & Grand Sport X Launch Alongside All-New 535hp LS6 V8!
Michael S. Palmer
5 Reasons Bad Drivers Crash & 5 Ways to Avoid a Costly Mistake!
Joe Kucinski
7 Bolt-On Upgrades From Extreme Online Store to Level Up Your C6 Corvette
Pouria Savadkouei
Safety Car
Joined: Nov 2008
Posts: 3,705
Likes: 550
From: Seattle Washington
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.
Burning Brakes
Joined: Oct 2003
Posts: 1,097
Likes: 0
From: Olive Branch MS
St. Jude Contributor
LOL...How did you know my buddy has a minivan?
Thread Starter
Intermediate
Joined: Jan 2009
Posts: 31
Likes: 0
From: Southlake Texas
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
Burning Brakes
Joined: Oct 2003
Posts: 1,097
Likes: 0
From: Olive Branch MS
St. Jude Contributor
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
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
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
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
(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 )
