Simon928

I've been having a bit of a nightmare trying to get my brakes bled on my 1990 S4. My brakes were working perfectly, but since I bought the car last year I figured that I should get the brakes bled because I had no idea when they were last done. Since I bled the system my brake pedal is not what it used to be. The pedal is now as hard as a rock when the car isn't turned on, but as soon as it's turned on the pedal travels a lot more than it used to. The pedal doesn't go to the floor and the brakes do still stop the car, but the pedal has way more travel than it used to.

The first time bleeding the brakes was with the two man method (guy sitting in car pressurizes the brakes and holds the pedal while I open bleeder, I close bleeder and guy in car lets off of pedal). The first time I did it I didn't bleed the brakes in the correct sequence, but each subsequent time I've done the MC first, then FR, FL, RR, RL.

I also noticed that there was a leak at the grommets where the MC meets the reservoir. I'm not sure if the leak was there before I first bled the brakes or not, but I just replaced both grommets and there is no difference. I have bled the brakes at least 10 times now trying a bunch of different methods. I've used both the power bleeder and the two man method, I've jacked the front up higher when bleeding the front and the same for the rear, I've also removed the calipers and held them high and rotated them to try to get bubbles out. I've also compressed the pistons on the calipers before putting them back on with the hopes of releasing any bubbles that might be trapped. None of this has had any effect on my symptoms. Brake pedal is always hard when the car is turned off and soft when I turn the car on.

I suppose that the next thing for me to try is to replace the master cylinder. I've avoided replacing that until now because I was under the impression that if I held pressure on my brake pedal and it stayed firm then the MC shouldn't be leaking. Are there scenarios where this isn't the case? I'm more than willing to replace it if that will fix the problem, but at the same time I want to avoid throwing money indiscriminately at a bunch of different parts if they will have no positive effect.

Any input would be appreciated! If I had any hair left on my head I would've pulled it all out by now.

SwayBar

Are you bleeding both bleeder screws per caliper?

Simon928

Yes, I bleed the inner bleeder screw and then the outer for each caliper.

Mrmerlin

Based on the info provided your working great MC has now been damaged due to bleeding.

The seals were pushed over unused parts of the bore.

PLEASE follow these instructions and you will have great brakes again.

put in a new ATE master cylinder,
use Gold ATE brake fluid,
get Dow Corning 111 and put it on the inlet grommets and the MC to booster gasket
(this will prevent the MC inlets from corrosion)

Wash the tank with brake cleaner,
then simple green and hot water ,
then rinse with alcohol,
then blow dry with a hairdryer.

Check the booster for fluid contamination from you old MC,
replace it if fluid has gotten into the booster.

Replace the booster check valve.
Replace the blue line now a black line for the clutch feed
Replace the rubber lines at each wheel with stainless flex lines,
Roger has them

Simon928

Thanks Stan! I read some posts that you made in similar threads about the rusted MC piston cutting the seals, but I was under the impression that if the brake pedal didn't drop to the floor under pressure then the MC was probably okay. Just to counteract my own admitted ignorance, can you let me know why that isn't the case? I'm not doubting your diagnosis at all as I'm about to put an order in on a new MC as we speak, but from what I read about my brake pedal firmness I was told that it would rule out a bad MC.

StratfordShark

My feeling on this is that with no vacuum assist you are pressing hard enough on pedal to flare out the piston seals so the pedal feels firm. With engine running and servo helping you are applying only light pressure, so the damaged MC seal is not flaring out but is allowing fluid past, so pedal is sinking.

Mrmerlin

the only way to prove this is to take the master cylinder apart and inspect the seals and bore.
the MC is made to not be able to be taken apart easily,
so most attempts to dismantle it will damage the piston, thus a new MC will be needed.

You have already provided facts about the performance of the MC you have ,
the rusted grommet bores cinched it.

Their is not any way to rebore the grommet seats unless you have the tool that made the cylinder in the first place,
then you would have to make sure that the swarf didnt get into the bore.

Since you have no data on the POs treatment of the MC and its probably original.
please just follow my suggestions, and the added parts to install in the earlier post

Simon928

Thanks Stan--I already ordered the parts so I'm not doubting your diagnosis, I'm just trying to fill in the gaps in my knowledge as there are many. As I mentioned I was told that a hard brake pedal usually meant that the MC was *probably* okay, so I was just trying to understand how the MC worked and why it could still be messed up yet still have a hard pedal. StratfordShark posted an interesting theory, so if that is indeed the case then that could explain it.

Thanks for the tips everyone!

Mrmerlin

please let us know how things turn out, thanks

Simon928

Will do! I know how to remove the reservoir, and when it comes to removing the MC I see two hard lines to disconnect and two bolts against the booster. It looks straightforward...are there any "gotchas" that I should look out for?

StratfordShark

One of those bolts against booster is very hard to reach, but is doable. I haven't had to myself, but I understand a good selection of 1/4" extensions helps!

Mrmerlin

I use a short 13 mm wrench with a box on one end ,
its easy to remove the the nut just takes some time.
Put a couple of rags under the MC so you dont drip onto the cam cover.
spray the reservoir with PB blaster at the grommets so the tank will easily slide out

Simon928

Well I just replaced my master cylinder with a new one and bled the car a bunch of times, and there isn't any change in the softness of the pedal. The MC is all that I have changed so far, as I want to eliminate one thing at a time instead of replacing everything at once.

I guess the next thing to investigate is the brake booster. Stan mentioned that I should look at it to see if any fluid has contaminated it. I pulled what I assume is the check valve and stuck a wire all the way into the bottom of the booster and fished it around a bit, and then pulled it out to see if there was any sign of fluid on it. It was dry. Is there anything else that I should check with regard to the booster? I'll be ordering a new check valve just in case.

While doing some looking around I see that Nicole had a similar experience with her brakes and the solution for her was replacing her brake bias valve. Is there a possibility that it could be a culprit for me as well?

And speaking of the bias valve, I can see that the lines going up to the ABS & bias valve don't have any bleed screws. If I have the lines under pressure with my power bleeder (~10-15 psi) can I crack open the nuts on the lines around the ABS in order to get rid of any air that might be trapped in there, or is that a bad idea?

Mrmerlin

lets go back here ,
how did you bleed the MC?

Heres what to try next.
A new MC will usually precede air getting into the lines.
More than likely you have air at the ABS unit.
SO fill the res,
then open the two B nuts one at a time till you see no more bubbles,this is a gravity bleed.
NOTE work from the booster forward.

then open the tip bleeder. make sure no air.

Then move to the ABS unit.

NOTE if you have pressure bleeder that screws onto the reservoir then use it no more than 10 psi.

What your looking for are air bubbles to come from each B nut connection at the ABS unit,
open one at a time .

Then remove the pressure bleeder.

Go back to the MC have the helper press the pedal slowly,
and crack the B nuts one at a time to verify the air removal, do the tip last,
then move to the ABS unit,
crack each line, with the helper on the pedal go slow during this process.

then bleed the wheels LF RF LR RR. This should remove the air in the lines.

what happened was your old MC failed,
then you continued to pump air and fluid into your system.
Please report back,
BTW where did you get the MC did it look new no rust?

Simon928

The MC was brand new--I got it from Roger. Regarding how I bled the MC, it was done in the car through the bleeder screw. I also got two significant air burps through the RF inner bleed screw, so I assume that air was from the MC as well. That of course means that air got into the system and could have got into the ABS lines too, so thanks for the tips related to bleeding those. Just for full disclosure, after I installed the new MC I bled the car at the MC and all calipers twice and then took the car for a drive. That was last weekend--yesterday I took the wheels off and bled everything again twice, and I think I got two bubbles out.

Someone mistakenly cracked the cap of my power bleeder while it was on my work bench so I ordered a new cap, but it might take a week or more for me to receive it in the mail. Where you describe bleeding the ABS with the power bleeder, can I achieve the same thing with someone pressing on the pedal by cracking the lines open while the pedal is pressed and then closing the nut before the assistant lifts off of the pedal? Or should I just wait until I get my new cap in the mail and be safe?