Help needed with brakes on my E9 3.0 csi

[Goose]

Hello all.
Any brake experts here that can help me out with some ideas?
When I press the brake pedal, it does not brake; it goes to the floor, then it breaks a little.
Please read the entire story below since I tried a lot of things already.

Everything was working fine, the car was braking correctly until I noticed an unusual amount of brake dust on one of the rear wheels. Lifted the car and found out that the wheel was very stuck because of the pistons. So I ordered a rebuilt kit for both rear wheels and fixed that issue. At the time of bleeding the system, I noticed that no brake fluid was coming out of one of the rear calipers. That's when I realized that the two rubber hoses on the back have not been replaced with metal ones (the front ones are metal already). So I replaced the hoses and brake fluid come out from the calipers.
Now this is when my current problem started. From here, I could not get the brakes to work. After several bleedings, including using a pressure bleeder. I can't get the brakes to work. They go all the way out to the floor and then it breaks a little.

Assuming that was a vacuum leak, I first replaced the return valve. Nothing happened, same problem.
Thinking that there was a leak on the booster, I sent it to be rebuilt. installed and the same problem.
Then I bought a new master cylinder. Same problem.
BTW, no fluid leaks anywhere in the brake system

I'm out of ideas and options. Here are some interesting facts:

-When I press the brake pedal, the fluid in the reservoir goes down noticeably. When I release the brake, it goes up again. Could this be air?
-When I have the pressure bleeder connected to the reservoir, with pressure, the wheels do not rotate, it is like if they have brake pressure applied without pressing the pedal. If I release the pressure on the bleeder and press the brake pedal with the car ignition off, I can rotate the wheels with my hand, there is no pressure on the brakes stopping the wheels.

My next thing to check is to remove all the seals and pistons on the front brakes, but I don't know if that is going to do anything.

Any ideas or recommendations will be appreciated.

best.

ReplyForward

 

[Barry.b]

Hi There
I think it could be pressure proportional valve that is causing you some problems.
In theory if it is stuck or sticking, you could have an air pocket trapped between it and your brake booster. As air is compressible it may not be able to pass / overcome a sticking pressure valve and then not be able to get to the bleed nipples on your rear callipers.
It might be worth trying to see if it is functioning properly.

Rear brake pressure regulating valve Rebuild

Hi guys I just rebuilt my rear brake pressure regulator valve. I used a pmb kit that is meant for an early Porsche. Anyway I took mine off and cleaned it and glass bead blasted it before I took it apart. It was easier to keep the blast media out that way . It is spring loaded so I put it in a...

e9coupe.com

 

[JFENG]

pressure bleeder connected to the reservoir, with pressure, the wheels do not rotate

The pressure bleeder is acting as a surrogate for your master cylinder. It will ‘apply’ your brakes even if your MC is bad and/or you have lots of air in the lines.

When you replace your MC it can take quite a bit of bleeding to get all the air out. i needed nearly 2-quarts to do an old Jag a few years ago. How much fluid did you bleed thru each rear caliper after you replaced the MC & flexible hoses? Did you pre-fill the MC & pistons per Bavbob’s post?

Here is a quick test you can try to help determine if you have air in the system. Apologies in advance if your already tried all of this.

Pump the brakes 3-4 times in rapid succession. If this causes the brake pedal to engage higher off the floor, then it is fairly likely you still have air in the lines. Bleed the system more thoroughly (follow brake bleeding best practices).

If pumping the pedal rapidly does not help bring the pedal off the floor then I would suspect a fault with the MC (seals).

John

 

[bavbob]

John, he replaced the MC.

I usually pre-fill my calipers after rebuild before I put them back on he car. Syringe comes in handy, faster bleeding.

 

[Dick Steinkamp]

I generally advise against firing the "parts canon" at a problem (that is...replacing lots of parts hoping to get lucky and changing one that solves the problem), but in the case of brakes with an unknown history, it makes some sense. If your car doesn't go, it's inconvenient. If it doesn't stop, it's deadly.

If your rear hoses were old enough that they wouldn't pass fluid, the rest of the system would be suspect to me. The front hoses (and the rear ones you used to replace the existing ones) are not metal. They are generally a rubber substitute wrapped with a metal mesh. They are that way primarily for the purpose of giving a slightly firmer brake pedal feel. They can still age and swell internally like the rear ones did. If you don't know the age of them, I'd replace all 4. If you don't know the history of the front calipers, I'd replace or rebuild them also. Same goes for the valve that limits brake pressure to the rear brakes to prevent the car changing ends under hard braking.

Here's a thread on brake bleeding...

Bleeding Brakes - Epilogue

The Haynes manual only has generic bleeding instructions. Ones that apply to most any car. It doesn't address the 3 bleeder screws on each front caliper. What is the proper way to bleed these front calipers? Thanks.

e9coupe.com

There are some differences of opinion on how do do it, so take your pick.

 

[Ohmess]

I got interrupted in the process of replacing my rear brake hoses, and after I got the car back together was unable to bleed the rear brakes.

It is pretty well known that over time, the brake lines in our car can break down. Sometimes they will swell as noted above, and sometimes they allow small particles of rubber into the brake system. In my case, those small rubber particles that had been inside the brake lines migrated up to the brake proportioning valve, clogging it off entirely. I allowed this migration to occur by leaving the rear brake lines open for too long. I think the particles could have been bled from the system had I plugged the rear lines when I left the car disassembled.

In my case, I had to replace the brake proportining valve before I could get the rear brakes to work.

 

[Goose]

Thank you all for the help.
This is giving me some ideas. There are not many things left to do aside from more bleeding and checking the pressure valve.
Replacing the MC and updating the booster was something I want it to do anyway since it makes it more secure. Now I just need to track this problem.