Sinking brake pedal (audi 100/200)

[Zachry]

Hi.

I just bought a Audi 100 (1984) that has been converted to a Audi 200 TQ (1987).
I bought it cheep because the brake system was shot and interior was falling apart.

The car was owned by an old man and he drove it to a EU-check (MOT you call it?) in January this year. The car failed because of rusty brake disks, uneven brake effect and a faulty "load adjustment valve" (the thing on the rear axle that adjusts brake pressure to the rear). All good. Its just labor and some costs in parts.
But when i bought the car in july now it had been standing since january.

Now the brakes had about no effect at all. I had to pump the pedal like crazy for a long time before it started getting any effect. The pedal was just soft. Since then i have been using the brakes (moving it around and stuff) and it seems everything with the brake system is "ok". This must be something with the brake assistant (hydraulic pump and accumulator "bomb").

So this is the deal: I pump the brakes: The pedal gets hard. I have brakes. But the pedal instantly begins sinking and brake effect goes away (with constant pressure on the pedal).
Must not forget the insanly irritating brake warning that comes up in the display accompanied by a beeping sound. This is the warning of low pressure in the hydraulic system i belive.

I have never worked on cars with hydraulic brake assistance, but the normal vacum kind ive done plenty.
Some key points:
-This is not air in the brake lines. Then i would have a spongy pedal, not sinking pedal.
-Its not leaking brake fluid.
-I cant find and hydraulic fluid leaks (not loosing any fluid from the reservoir either)
-I changed the "bomb" from a parts car. I was promised the car had ok brakes before.
No change.
-Power steering works fine (which indicates that its not the hydraulic pump).
-This happened (supposedly) while the car was in storage


Whats up? I dont understand this I cant seem to find anybody online who has had this problem. Anybody have any ideas, experience? anything?

I havent been able to confirm this: But it might feel like the brake doesnt sink (or not as fast) if the car is above idle. Will check this out tomorrow.

Kind regards Chris

[mikes2]

sinking pedal is a brake master cylinder

[Zachry]

Changed master cylinder today. Same result.
Any other ideas?

[TLRogue]

could be just because you have fitted new parts like pads and discs.

There was an mot bulletin a few years ago about vauxhalls having brake pedals that sink badly after having new pads and discs fitted. if it feels safe enought to drive, i would bed the brakes in properly and see if they improve.

[mikes2]

possibly the servo if the master cylinder has already been changed.

[Alpha]

Your loosing preasure somewhere..if youve changed the master cylinder for a new or good one then the only explination can be you have a leak, either corroded/ holed line, split flexi or leaking cylinder/caliper...

[Zachry]

Hi guys. Thanks i have not yet fitted new brake pads and discs. I have been troubleshooting and doing diagnostics all weekend. I now have brakes that still "sink" but are much better. Thanks to changing the master cylinder and bleeding some more.

But i have absolutly no power assist if the engine is off. The pedal is hard as **** the second the engine stops. This makes me belive the "bomb" from the parts car was dead. He is by far a audi expert and didnt test it while the parts car still ran. I was under the belief that if the bomb was dead you would have very bad brakes while the engine ran also. But you have some brakes. I asked the guy: "Did the brakes work good". "Yeah they were ok".
He only bought the car to have something to crash around at his farm.

So im ordering a new bomb from CAC now and will see how things turn out.
Anybody agree/disagree that this might be better if i get a new bomb? I can lock the wheels on asphalt now, but have no pumps on the pedal after the engine is dead.

And on another note: How do i find parts numbers on brake disks? My car is an audi 100 rebuilt to a 200. Im not sure what year/type my brake disks are and couldnt find any original parts number in the rusty old disks. Any other way to identify them to order new?

[Zachry]

Resolved. Problem is not certain how.

Alot of work has been done since i last posted. New bomb arrived. Didnt fix the sinking pedal.
Anyways i had to change all 4 discs and pads. And also the load adjusting valve.
Found some brake lines that were dodgy at best. Might have been those being the cause. But i didnt loose much/any brake fluid, but the lines were wet.
Everything back together with new brake lines and no more sinking pedal!!

Now to the next case: ABS!
I dont get any warnings on the ABS whatsoever. The only thing is the ABS kicks inn even when im not close to locking the wheels. I wouldnt really mind (just turn it off), but i have to go trough a road worthiness test soon. I guess they wont like that the abs is acting up.
After reading around online its pretty obvious that this must be a abs sensor acting up.
Everybody explains about cleaning them, but i havent seen any pictures.
Does anybody have a guide? Do's and donts of removing abs sensors.
Does it have 4x abs sensors or only one on each axle (havent looked for them)?
Do i need any special tools?

[saladdodger]

Cant believe after all the searching and time ive spent on my car ive only just seem this thread.

I have exactly the same sinking pedal problem