Hissing sound when braking

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Hissing sound when braking

March 24, 2012, 11:44:00 PM

I have a hissing sound when driving my car when braking. I replaced all the brakes on my sport coupe with turbo coupe brakes and ran new lines with an adjustable proportioning valve. The hissing sound was followed up with a brake peddle that went all the way to the floor with no pressure. I was told it was master cylinder and that if it was the vacuum pump I would have the opposite problem, the brake pedal would be very tight and I would not be able to press it down. After replacing the master cylinder and bleeding the brakes, I now have brake pressure but this hissing sound still prevails. If I press on the pedal when the car is not running, there's no hissing sound and the pedal is tight. When the car is running and I press the pedal down, the hissing sound is present and I still have pressure. Do I now need to replace the vacuum pump and bleed all over again or is there possibly an easier solution to eliminate the hissing sound? Any help would be greatly appreciated.

Hissing sound when braking

Reply #1 – March 25, 2012, 12:38:15 AM

The hiss is the vacuum dump valve...which kills your cruise control if it is on and you hit the brakes. You'll always hear SOME air hiss, shouldn't be constant, maybe the valve is sticking/worn/failing in some other fashion.

Hissing sound when braking

Reply #2 – March 26, 2012, 09:38:43 PM

I think the valve may be failing then because the hiss is constant, like there's air in the lines. I assume that means I need to replace the vacuum pump and in doing so, re-install the master cylinder and bleed the brakes again. Does anybody have any tips to make sure I don't do more work than I have to, or any thoughts on how to double check the problem (i.e. disconnect the dump valve first and see what happens). Thanks again in advance.

Hissing sound when braking

Reply #3 – March 27, 2012, 08:06:54 AM

Wait, how is the pedal during driving? As if there's no vacuum boost at all? Very hard pedal? Or is everything normal aside from that hiss?

Hissing sound when braking

Reply #4 – March 27, 2012, 04:27:32 PM

Check the vacuum line under the dash that connects to the cruise control valve. Make sure it is connected and not dry rotted.

Darren

Hissing sound when braking

Reply #5 – March 29, 2012, 05:50:20 PM

I'm not home but will check the hose next time I have a chance. The pedal is normal; it's just the hissing sound.

Hissing sound when braking

Reply #6 – March 29, 2012, 07:37:28 PM

Another thing just in case....if you have the Automatic Parking Brake release, that releases it when you shift from Park through Neutral into Overdrive, Drive, or 1, that might be hissing through the valve controlled by your shift lever. Wiggle that and see if the sound is affected at all. My crown vic got like that not long after I bought it, I KEEP FORGETTING, SOMEHOW....to just plug the line because it's more simple-minded for me to just wiggle it every time I have to move the lever and it just about seals up and goes away :hick: LOL

Hissing sound when braking

Reply #7 – March 29, 2012, 07:55:32 PM

Don't rule out a leaking booster.

Hissing sound when braking

Reply #8 – March 30, 2012, 01:18:20 AM

There should not be vacuum to the dump valve unless cruise is turned on.
If cruise is on, braking should put it in standby mode and shut off the vacuum.
If you find that it is the cruise dump valve hissing, DO NOT DRIVE THE CAR.
The car could run away.

Hissing sound when braking

Reply #9 – March 30, 2012, 05:43:01 PM

VACUUM BOOSTER. PLAIN AND SIMPLE. Seen it many many times. Just a thought. Easy to check with a length of 3/8 hose and your ear!!!