Brakes Hissing A100

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 inspection.
Thanks, Gary
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Isn't the bomb hissing? It hisses after starting the engine for a second which is normal.
Ronald
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Ovalking wrote:

Check the hoses to the brake servo unit. It uses engine vacuum.
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Haven't noticed that before, and it seems to be coming from the pedal area. Will grab an assistant for further check on this.

All hoses & pipes appear ok after a further inspection today. Actually the servo unit is hydraulic. I believe the vacuum servo was only used on the non-PAS (1.8?) models.
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writes:

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!
G.
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G, 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. Steve Sears 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. (SPAM Blocker NOTE: Remove SHOES to reply)

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Thanks for your comments and ideas Tony & Steve.

There's no cruise control - reserved for the posh models.

Yes I had tried this - I get about 1 'normal' press! It was like it when I bought it but didn't know about the test then :-)

No, not at this stage - only does it when the engine is running.

they're fine - recently replaced.

No - you don't have to be moving.

Checked with the MoT man and he said it would fail so have taken this 'jinxed' car off the road for a rest!
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Is your cruise control working? If not the sound may be a bad vacuum switch for sensing pedal movement to shut the cruise off. I assume the A100 has these but not sure.
Tony
Ovalking wrote:

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