Braking balance

Qwodracer · Post by **Qwodracer** » Sun Jul 10, 2016 6:11 pm

It seems like in emergency situations or spirited cornering, my front tires lock up long before my rears start to work. N the rain, light braking locks the fronts and I slide to a stop.

Is this common? I looked through my Haynes and couldn't find a balancing/proportioning valve... Is there one? Would make sense if it was bad.

Or is there just a problem with my back brakes? Brake pedal is high and hard, no air, good fluid.

All my pads are half way and rotors were rusty when I picked up the car so I'm saving pennies for a brake job down the road

JodyStevens · Post by **JodyStevens** » Sun Jul 10, 2016 7:02 pm

Bias valve is located right above the frame rail below the brake booster on the driver side(assuming LHD). Not sure if they go bad, but they are not serviceable or adjustable.

RandyM · Post by **RandyM** » Sun Jul 10, 2016 9:19 pm

The brake pressure regulator is heavily biased to the front, like 70 or 80% if I recall correctly

Mike W. · Post by **Mike W.** » Mon Jul 11, 2016 12:23 am

I've locked them up once or twice and found proportioning to be about right. Your description of the brakes rings an alarm though, mismatched pads even, let along odd rotors can throw things off. If you can find one easily a used one might be easier and cheaper, but pads/rotors are my guess. At the worst, fronts locking up first are better than the rears and the back end coming around on you. Not that I'd know anything about that mind you...

Qwodracer · Post by **Qwodracer** » Mon Jul 11, 2016 7:00 pm

well I would like it more balanced in the corners, right now I can only get sideways powering on or flicking it into the corner, and if I touch the brakes at all, the front just washes out, making it not very "safe" in relative terms 9if sideways were safe).

I've never had or driven an e12, but have owned a few e28 and drove many e30, e36 , maybe I'm comparing apples to oranges?

hilux30 · Post by **hilux30** » Mon Jul 11, 2016 8:08 pm

E12s are front brake biased but if you are locking on a "light braking", check: hard lines going back if anything is pinched, soft lines can also internally collapse and restrict flow, check fluid for contamination could cause seals to swell and quick lock up, but usually with contamination from water/chemicals ending up in lock-up with little to no release on all fours... assuming tires, suspension and pads/rotors are all good... otherwise a manual proportioning valve is always an option.

RonDwyer · Post by **RonDwyer** » Fri Jul 15, 2016 10:10 am

I have found a complete rebuild of all 4 calipers is helpful if there is no recollection of this being done in the last 20 years. Some of the pucks just get frozen in place which renders that one inoperative and throws the balance totally off. There is nothing like the first drive on a set of fresh calipers. Totally messy job however but easy enough for just about everyone.

I did a post a while back on my technique to free up stuck pucks on the car. Sometimes it is better to get a set at the junkyard, rebuild them first and just do a weekend swap. A power bleeder was one of the best inexpensive tools I ever bought.

Robert Bondi · Post by **Robert Bondi** » Wed Jul 20, 2016 5:21 am

RonDwyer wrote:I have found a complete rebuild of all 4 calipers is helpful if there is no recollection of this being done in the last 20 years. Some of the pucks just get frozen in place which renders that one inoperative and throws the balance totally off. There is nothing like the first drive on a set of fresh calipers. Totally messy job however but easy enough for just about everyone.

I did a post a while back on my technique to free up stuck pucks on the car. Sometimes it is better to get a set at the junkyard, rebuild them first and just do a weekend swap. A power bleeder was one of the best inexpensive tools I ever bought.

I just did some brake work partly aimed at correcting pulling while braking. Despite having both brand new front BMW calipers, I actually still have a pull to the left after a test drive. I took it apart again to notice different responses from all the pistons in the left caliper rather than fairly even operation from the right caliper.

Given the three bleed screws on the front calipers, I've started wondering if some uneven bleeding could cause different response among the pistons or is it just a matter of lubricating and exercising the lazy pistons back into shape?

I need to look up your recent post referenced as I revisit this problem. I've never actually had a seized piston, but I often seem to be in the situation of different response times of the front pistons causing a pull in one direction.

PatinaBeforePolish · Post by **PatinaBeforePolish** » Wed Jul 20, 2016 6:43 am

"Different responses" between calipers indicates semi-seized piston or pistons. But these are new OEM from BMW? They come with a 2 year warranty if defective. How did you measure the response? How dramatic is the "pull"? If it pulls hard, then likely caliper / pad / rotor related. A mild pull could be worn suspension bushings, unequal tire pressure, mismatched tires or suspension misalignment.

Like hilux30 mentioned, I also would strongly consider replacing all 6 brake lines. I use teflon/stainless lines for peace of mind and a slightly better braking feel. They're available from many sources. I agree a power bleeder is an excellent investment that can be used in a lifetime of car maintenance. The bleeding sequence should always be from the lowest in elevation bleeder to highest. Lightly tapping the brake caliper helps bring the bubbles to the top and you may have to bleed, drive and bleed again. I flush and replace brake fluid (and coolant) every fall in all of my cars. Cheap insurance.

RandyM · Post by **RandyM** » Wed Jul 20, 2016 9:37 am

What was the wear pattern like on the pads? I had a a blown seal on a front piston. The car pulled to the other side and the pad with the offending piston was thicker/less worn than the pad on the opposite side of the rotor.

Robert Bondi · Post by **Robert Bondi** » Thu Jul 21, 2016 5:35 am

PatinaBeforePolish wrote:"Different responses" between calipers indicates semi-seized piston or pistons. But these are new OEM from BMW? They come with a 2 year warranty if defective. How did you measure the response? How dramatic is the "pull"? If it pulls hard, then likely caliper / pad / rotor related. A mild pull could be worn suspension bushings, unequal tire pressure, mismatched tires or suspension misalignment.

Like hilux30 mentioned, I also would strongly consider replacing all 6 brake lines. I use teflon/stainless lines for peace of mind and a slightly better braking feel. They're available from many sources. I agree a power bleeder is an excellent investment that can be used in a lifetime of car maintenance. The bleeding sequence should always be from the lowest in elevation bleeder to highest. Lightly tapping the brake caliper helps bring the bubbles to the top and you may have to bleed, drive and bleed again. I flush and replace brake fluid (and coolant) every fall in all of my cars. Cheap insurance.

Yes, these are new BMW calipers. The new parts on the recent brake work:
- new rotors and pads on all 4 wheels
- new front calipers (rear are about 8 yrs old)
- all 6 flex lines are braided stainless from Ireland Engineering

The different response is just a visual assessment. With brake pads removed on the left caliper and right side with pads still in or blocks of wood to block the pistons, I went and pumped the pedal a few times then checked the piston motion. The pistons in the left calipers all have different effective resistances. By consecutively blocking off each piston with the least resistance, I can eventually get all pistons to move by repeating that procedure until only the most reluctant piston is unblocked.

The pull was pretty severe on the initial test drive when braking hard.

I use a power bleeder too, but I find it still needs a little bit of pedal bleeding with a helper because of weird behavior with the master cylinder. Also, it always seems like a drive is needed an a rebleed. I always do a little bit of tapping with a rubber mallet or dead blow hammer while bleeding too.

I was just reminded in some recent posts that neither the bleed sequence you mentioned nor the one I used is correct for the front calipers. The factory manual suggests this bleed order: outer upper, inner, outer lower.

Still wondering if I should get another caliper or if I can lubricate the pistons for more even response.

Robert Bondi · Post by **Robert Bondi** » Thu Jul 21, 2016 5:38 am

JodyStevens wrote:Bias valve is located right above the frame rail below the brake booster on the driver side(assuming LHD). Not sure if they go bad, but they are not serviceable or adjustable.

Yes, the pressure regulator can go bad. In my case, it just started leaking.

Robert Bondi · Post by **Robert Bondi** » Thu Jul 21, 2016 5:47 am

RandyM wrote:What was the wear pattern like on the pads? I had a a blown seal on a front piston. The car pulled to the other side and the pad with the offending piston was thicker/less worn than the pad on the opposite side of the rotor.

Randy,
There's not enough mileage to establish a wear pattern. After one 30 mile or so test drive after finally getting the pedal firmed up, it's up on jacks again because I was so unhappy with the pulling issue.

The front pads might also be defective. They're new Jurid pads from BavAuto. Somehow, pieces have managed to crumble off the edges and some have embedded themselves back in the grooves on the pad. I've never experienced that before. The pads were supposed to be somewhat higher performance that stock BMW. I'm not impressed. I ordered so new Ate pads and will seek a refund.

PatinaBeforePolish · Post by **PatinaBeforePolish** » Fri Jul 22, 2016 3:29 pm

Robert,

The blue e12 factory manual section 34-00/2 says "Bleed all 3 bleeder screws A, B and C. Always bleed both lower pistons in sequence B-C." I interpret that as lowest to highest in elevation. (A is the upper screw, B inner and C outer)

I know we're drilling down to tiny details here - grease on the brake pads or rotors? Have you bedded the pads?

RonDwyer · Post by **RonDwyer** » Fri Jul 22, 2016 6:12 pm

A power bleeder is essential for these projects. A few PSI pressure at the reservoir and all you have to do is open the valve and wait for the bubbles to stop.

Stainless teflon hoses...very popular, and not DOT approved for one reason, the teflon hose shatters and will not absorb a debris impact or a 90 degree flex. Having worked for Aeroquip years ago, we had to answer this question all the time. Race Only.

A brand new set of ATE hoses work just as well for highway use. A chunk of a shredded truck tire is enough to ruin one of the teflon lines.

As for bad balance, I hone my caliper housings when I rebuild them.