Is it normal to hear a slight "hissing" sound when partially depressing the brake pedal that stops when fully depressing it? This is on my '87 Topaz and I am STILL trying to determine the reason for the idle issues. I would assume it has a dump valve near the brake pedal but I haven't had the time to look. Don't want to trace it down if this is normal. By the way, these are power brakes and I have replaced the vacuum line and check valve going to the brake booster. The booster itself, however, is the original. Anthony
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To me the hissing sound has always been the indication of a bad booster.
I've had to replace two boosters on two different cars because of the hissing sound. -
Surprising, though, that it stops when your foot is completely down. Could it still be a bad brake booster??Comment
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Probably the booster. It can go a very long time with a slight hiss. Just pay attention to brake effort, when it starts to get tough, time to swap it. I had a hisser on my truck for many years before finally changing it out.
Alex.Comment
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Wonder what effect it would have on a somewhat erratic idle??? I would think if the diaphragm in the booster is defective it would act like a vac leak because that is what it really is.Comment
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the booster in the car when I got it did that, hissed as depressed but not when you're holding the brakes. I thought it was an issue until the one I put in to replace it did the exact same thing.
You can hear it in this video, first when I let off to launch, and then quick shots as I brake for each corner (at least the ones I braked for).
85 4 door 351 Civi Crown Victoria - Summer daily driver, sleeper in the making, and wildly inappropriate autocross machine
160KMs 600cfm holley, shorty headers, 2.5" catted exhaust, 255/295 tires, cop shocks, cop swaybars, underdrive pulley, 2.73L gears.
waiting for install: 3.27's, Poly bushings, boxed rear arms, 2500 stall converter, ported e7's, etc
06 Mazda 3 hatch 2.3L 5AT (winter beater that cost more than my summer car)Comment
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Yeah, you can hear it! Mine is very subtle - lower than yours and the brakes work great. As I am trying to run down an erratic idle problem and still haven't found it yet, I thought this might be a possibility. That is what I am trying to find out.Originally posted by johnunit View Postthe booster in the car when I got it did that, hissed as depressed but not when you're holding the brakes. I thought it was an issue until the one I put in to replace it did the exact same thing.
You can hear it in this video, first when I let off to launch, and then quick shots as I brake for each corner (at least the ones I braked for).
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if you want to eliminate it, just pinch or plug the line to the brake booster.
85 4 door 351 Civi Crown Victoria - Summer daily driver, sleeper in the making, and wildly inappropriate autocross machine
160KMs 600cfm holley, shorty headers, 2.5" catted exhaust, 255/295 tires, cop shocks, cop swaybars, underdrive pulley, 2.73L gears.
waiting for install: 3.27's, Poly bushings, boxed rear arms, 2500 stall converter, ported e7's, etc
06 Mazda 3 hatch 2.3L 5AT (winter beater that cost more than my summer car)Comment
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Forgot with all that has been going on that I plugged the vac line going to the BB and it didn't change the idle. Guess I'll have to keep looking. Is there a brake dump valve by the brake pedal? Tough question as it is not a GM or TC.Originally posted by johnunit View PostComment
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possibly, but if there is it would only be for the cruise control. Only has vacuum when the cruise is on.86 Lincoln Town Car (Galactica).
5.0 HO, CompCams XE258,Scorpion 1.72 roller rockers, 3.55 K code rear, tow package, BHPerformance ported E7 heads, Tmoss Explorer intake, 65mm throttle body, Hedman 1 5/8" headers, 2.5" dual exhaust, ASP underdrive pulley
91 Lincoln Mark VII LSC grandpa spec white and cranberry
1984 Lincoln Continental TurboDiesel - rolls coal
Originally posted by phayzer5
I drive a Lincoln. I can't be bothered to shift like the peasants and rabble rousersComment
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The car does NOT have cruise control. Guess I need to look elsewhere for the problem (or live with it).Comment
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