Being annoying me for a long time, I just dont think the pedal feel should be this soft. I know the brakes work well, but the pedal is very soft. Its also worse when the engine is cold and just starts. To me this sounds indicative of a brake bomb (accumulator) failure. Ive already changed brake lines, pads and flushed the fluid. Its topped up and fresh looking still.

Where is this part (the bomb) on the D2? Anyone change it already?

PS: Great photo of the brake boost pressure line here, mine looks fire.
https://www.audiworld.com/forums/sho...php?p=15532720

 

mine (2000) is a blue plastic triple sphere driver's side:


part is 443 131 541

 

Someone correct me if I'm wrong (and Bentley), but the brake booster works only directly off manifold vacuum (huge volume require through huge hose)...instant loss of boost when motor shuts off...no help at all from a vacuum reservoir. They are not even directly (indirectly maybe, using same vacuum source)in the same circuit with the brake booster and are for much smaller tasks (intake changeover pod(s)...storage WAY too little and lines WAY too small to help. The brake booster requires massive constant vacuum to work properly...motor running only.

Plus, any loss of vacuum to the brake booster leads to HARD/high brake pedal...hard and huge loss of brake power, with MUCH more effort required.

I think you're looking in the wrong place...sounds more like air still in the lines, failing hydraulic seals (or bore) in the master cylinder or something in the hydraulic circuit...NOT vacuum.

Many disagree with my next suggestion, because Ross Tech (and most mechanics) says it has nothing to do with it, UNLESS you have let the ABS hydraulic unit go at least partially dry.
...But...
If you have Vagcom (and have eliminated other obvious causes), it won't cost a penny >>> Run the ABS hydraulic unit bleed or function test (not sure how it's listed).
Some have had luck with solving some braking problems. One guy recently "cured" an inexplicable soft/low pedal by simply locking brakes to the point of ABS kicking in to pulse the pressure (as it's supposed to)...back to normal after that.
Maybe not "by the book", but there seems to be something in that unit that prevents full braking (in some cases), esp after brake work has been done.

 

with the car off, does the pedal feel mushy? If so, my money is on your master cylinder.

 

Ross tech all 4 wheels must be off the ground to do this.. is this true? Bit of pain if so.
http://www.ross-tech.com/vag-com/cars/absbleeding.html


To the other question, with car off the pedal is fine at first, getting harder with each press, after 5 it seemed rock hard. Which in BMW land means your brake "bomb" is bad..

 

I've never performed the Vagcom procedure...but to my knowledge, others did not all raise all 4 wheels. I don't believe it would hurt anything to run the test on the ground...sincerely doubt it could make things worse.

If you find no other problems and depending on the mileage of your car and it's past record (brake fluid changed correctly every 2 years?), a small amount of moisture (attracted to brake fluid) can settle in a nd eat the bore of a master cylinder.

I've only owned German cars in the Porsche/Audi/VW family...not a big fan of other makes and not familiar with most of their systems. Have NEVER heard of a brake bomb and never seen anything but direct manifold vacuum supply to the brake booster. Vacuum reservoirs on the D2 are not tied to the brake system, unless that info is completely missing from Bentley...or any repairs I've dealt with on any German brake system.

 

Ill go out and give it a shot now, 4wheels on ground.

Btw, on Brake "bomb", perhaps I didnt explain it correctly, do Audi's not have a similar system?

It augments your braking pressure and when the bomb (Hydraulic Accumulator) leaks internally you get a hard pedal very quickly when the engine is off and softer brakes when the engine is on. Which is what I got in the S8!

 

Argh, seems my PF model doesnt support the proper stepped through output test, just this basic ABS pump spin up test:

http://www.ross-tech.com/vag-com/cars/absbleeding.html

Will test on the commute tomorrow.

 

That sounds like systems used in older (very large) American cars that had full hydraulic power brakes, that would throw you through the windshield with a light touch....impossible to modulate with your foot and very irritating to use....have never seen similar on a modern German car (my brands)...maybe on MB fly-by-wire brakes(?) ...I dunno...not sure how that scary set-up works.

The only thing similar I know of (up through D2's anyway), is the extra hydraulic pump (maybe includes accumulator?) used in ESP...BUT that only uses stored (or readily produced) hydraulic pressure to apply single brakes for corrective actions NOT initiated with the brake pedal or master cylinder, rather from sensor input and supplied directly from the ESP pump to the needed brake caliper(s).

Modern vacuum boosters, master and caliper cylinders work so well and instantaneously that extra hydraulic boost has been (to my knowledge) unneeded...except (in D2's and other cars I own and have owned) for the self braking ESP function.

 

The type of system hes talking about was used on older audi 100's and V8 quattros. BMW 750iL and 850i cars used a full hydraulicly assisted system ran off the powersteering pump. SAABs had a accumulator with an electric pump for assist. As far as i know, these cars dont really employ that system.

According to All data. For assist there is the Booster that has a electric vacuum pump that will run under certain conditions. But any assist issues cause hard pedal. Yours is soft. So as said, Hydraulic problems. Also a caliper that has a broken bolt, or extremley worn out slides will cause a hell of a low squishy pedal.