The last couple of days I've noticed a hissing sound when I apply the
brakes, a bit like air rushing out of somewhere. Anybody know what causes
this? I couldn't find any references to hissing in searches.
Car is a UK 1988 Audi 100 2.0 Avant, I get the low brake pressure warning
lamp ocassionally but had put this down to a bad bomb (which I know needs
replacing). The brakes do work satisfactorily and the fluid levels are
all ok. There's no evidence of any brake fluid leaks from a quick visual
The hissing is definetly loudest from the pedal area inside the car and
it seems to be getting worse :-( You notice it most at the end of the
pedal stroke or if you hold the pedal down a while. As this is usually
when the brake pressure warning comes on I can only assume it is a side
effect of the bad bomb.
I can feel no leaks from the area above the pedal, and I have checked all
pipes & the brake calipers and everything appears in order.
Does a bad bomb fail the MoT? If so I might have to make this a parts car!
I doubt that it's the pressure accumulator aka "bomb" - the pressurized
nitrogen in there, a small volume (about the size of your fist) around
1200-ish psi, will not be rushing around the floor area of the car -
although the accumulator is in that area, it's on the other side of the
firewall with all of that sound deadening insulation there. The master
cylinder is also on the other side, so I doubt it's sound transfer into
there. Do the standard test - run the car for a few minutes, shut it off
and repeatedly press the brake pedal - it should act normally for about 30
presses, before becoming near-impossible to apply. If there's less, you
need a new accumulator (or refit it to be rechargeable). With the engine
off, and car motionless, do you hear the hissing when applying the brakes?
Maybe, as Tony pointed out, the cruise control vacuum pump is stuck on and
the pedal switches are vacuuming away when the pedals are pressed. Maybe
the pad warning pins are beginning to rub (have you checked the pad
thickness?) Or.....maybe.....the hissing is road noise and you have a
cracked firewall near the pedal mounts, letting in road noise when the
brakes are applied (heard of clutch area firewall cracks in 80's coupes)???
A bad bomb will result in almost no brake assist if your engine dies - not a
good thing on the highway, for sure. I had that happen once when the fuel
pump relay died - the brakes worked OK as long as the engine was still
turning - something accomplished by bringing the clutch out, letting the
engine rotate to drive the power steering pump (which boosts the brakes
through the accumulator) - sorta like an engine failure on a helicopter -
"automobile autorotation". I'm sure that an inspector would not overlook
the condition of the braking system.
1987 Audi 5kTQ - retrofitted bomb working well
1980 Audi 5k - good ol' vacuum brake assist...or is that new?
1962 and '64 Auto Union DKW Junior deLuxes - brake pressure assist = long
brake pedal lever.
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