Improving brake linearity?

Hello, everybody.

Now that I have my coupe in driveable condition, I'm focusing on improving certain aspects. One of these aspects is the brakes. Right now, the brakes are a very binary affair, meaning they feel like they are either on or off, rather than providing a more linear feel, like my 540.

Right now, it is running Super Blue fluid, and has fresh rubber connecting lines all around, as well as freshly rebuilt calipers all around, fresh pads all around, and new vented rotors all around. all being driven by a 2002tii booster. I figured steel lines would be first, but others have said that if the rubber lines are fresh, not to bother.

How much does your pedal travel before the brakes bite?

About 3/4ths of the way. As soon as you get that far, the pedal resistance shoots up and the brakes throw you against the seat belt. It will stop, not doubt about it, it just makes smooth driving something of a challenge. Before that, practically nothing happens.

There is a pressure regulator that is usually forgotten about (at least by me) that is supposed to keep the rears from locking up, have you noticed anything along those lines? I'm not sure if there is a way to test or adjust it.

The brake bias valve, by the diff? No, there hasn't been any locking up or uneven brake distribution, as far as I can tell.

I'll just throw this out there - you have a 3.5 engine / injection, is there enough vacuum in the intake manifold to operate the tii booster? also there is a valve in that line, could it be defective or leaking?

Sounds like a booster issue. It may just be a bad booster. Is it holding vacuum? Did it always act this way?

I had to replace the brake bias valve Tilla referred to. Mine was leaking internally, so the symptom was a soft pedal for the first pump or two until it built up pressure. I don't know what the symptoms would be if it is sticking internally - perhaps it could cause your symptoms. Carl Nelson sells rebuilt ones on an exchange basis.

This may be a long-shot, but what's involved with converting to the Tii booster? Is it a direct bolt-in using the Tii master cylinder -- or are there linkage issues to contend with? If the booster is good, there could be issues with the linkage and/or push rod length that affect the engagement of the piston in the master cylinder. If the rod is too short, the master isn't engaging as you push the brakes until the very end. If its too long, the piston doesn't return all the way and the brakes can drag. I've run into this on other cars, but I'm just not familiar enough with the Tii conversion to know if it could be your issue.

In my (now gone) Motronic powered e12 and in my e9, the brakes are very linear. Something is not right in your system, and I'm with the "it's the booster" crowd.

Hm, ok, I'll check the vac lines. It is very possible I have a vac leak somewhere. It has always acted this way since I got the car.

Right now, it has a CS master cylinder with a custom linkage rod. I think the linkage is the right length, as I can press the pedal all the way and the car locks up. It isn't dragging, either.

The booster seems to be working, as there is a world of difference between baking with the car off and braking with the car on.

Ok, there don't seem to be any vac leaks, and like i said, there is a huge difference in brake effort and feel when the car is on versus off, which would indicate that the booster is at least minimally functioning. What kind of tests can I do to verify the booster is in spec? Rebuild potential?

The easiest booster test is this -- get in the car, and before starting it, pump the brake a few times and hold it. Then start the car. The pedal should go "soft" as the vacuum in the booster builds.

Also, given that you are not getting braking until 3/4 pedal, you don't seem to have proper master cylinder function. You might try bleeding the master. Typical BMW power brakes begin to slow you within the first inch or so of pedal travel, and build resistance very linearly. I've had a 2002, an e3, an e9, an e21, an e30, and an e24 with vacuum assist brakes, and they all have very similar feel.

My car exhibits the same braking behavior. I am told that it is very difficult to get a good "pedal" in this model, by VSR1.

Pump the brakes 20 times and then start the engine with foot firmly on brake pedal.
If pedal drops the booster is working.

I'm thinking AndyM's response may be important here. I'm putting in a tii booster right now, and I have read where some have modified a CS push rod to install it with the tii booster.

I have a threaded rod that I obtained with the booster (I think from a tii) that will allow me to set the distance from the pedal to the surface on the booster where it mounts to the pedal box at exactly the CS spec, but I could see where someone cutting/welding/threading a rod to fit might not get this exactly right.

If you have no vacuum leaks, and Don's pump the pressure out of the booster test indicates that the booster works, and the problem persists, it would seem that checking the rod to pedal linkage distance would be next on the list.

Thanks Ohmess I do agree that at least in theory, the mechanical linkage should be adjustable to take up the play in the pedal.

Does anybody have information on how to adjust the brake pedal/linkage to take up the play in the system, without replacing parts? Is this a simple matter ?

Two things I'd try before throwing money at the situation:

One, the "Italian Tuneup" for brakes is five hard near-complete stops from 80 to even out the distribution of pad material on the rotors.

Two, I'd try a different set of pads (I didn't see mention of what pads you are using).

Either of these might do the trick, the first is cheaper and more fun than the second.

Anyone contemplating brake issues (that aren't simply mechanical) should read the materials on the StopTech website, very informative. I think one of them may apply to your situation with reference to suggestion #1 above.

http://www.stoptech.com/technical-support/technical-white-papers/-warped-brake-disc-and-other-myths

Neon, IIRC the brake pedal pushrod has a clevis that's adjustable. Be sure to leave a little play in the system at all times, just enough to feel.

My brakes are pretty linear, I only deviate from a straight line when I brake under the influence.

So, I have a question.

The rod threaded into the clevis rod end has a female end that threads onto a rod protruding from the booster, and on the opposite end it is threaded into a clevis rod end. The booster rod has a nut that butts against the female rod end, and the rod the clevis installs onto has a nut that butts against the clevis rod end.

When putting this together (two days ago), I set this up so that the distance from the mounting face of the brake booster to the center of the clevis rod hole was 14.45." I then snugged the booster rod nut and tightened the nut against the clevis rod end so that this distance cannot change.

The clevis rod end opening positions the clevis on both sides of the brake pedal body, and the clevis pin fits tightly into the clevis rod end, through the brake pedal body and out the other side of the clevis rod end. The clevis pin is retained by a clip that fits into a groove in the clevis pin, and when this is installed the pin and clip are tight up against the sides of the pedal body. Also, the holes in the clevis rod end and in the brake pedal body are only slightly larger than the clevis pin, holding the pin tightly.

I see where the clevis can be used to adjust the distance from the pedal attachment to the booster, but I don't see any opportunity to leave play in this setup. How do you leave play in the system?

Honolulu said:

Neon, IIRC the brake pedal pushrod has a clevis that's adjustable. Be sure to leave a little play in the system at all times, just enough to feel.

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