Front brakes locking up

[stuartinmn]

I just got my M535i out of the garage today for the first time since last fall, and took it when I went out for lunch. When I got home I noticed it was sluggish, and when I parked in the garage I figured out both front brakes are locking up - with the car in neutral I can't rock it back and forth, and the the rotors and pads were very hot (I have a blister on one finger now to show for it.) I'm hoping it's just rust or gunk plugging the lines and a fluid flush will fix the problem, but I suspect that since both front wheels are affected it may be a failed master cylinder. Having said that I didn't notice any change in the brake pedal feel.

Edited to add: I didn't think about how the dual brake circuits work - one front caliper and the opposite side rear caliper are on each circuit, so I'm not sure why both front calipers are sticking while both rear calipers aren't.

[Mike W.]

This may be more rambling than answers, but here we go.

On my 85, 535i US spec car, I think it was twice, I had the fronts lock up after sitting for an extended period of time with my foot on the brake. Not standing on it, just on the pedal, relatively level ground. Cellphone call so I pulled off the road. A hard push on the pedal freed them up. Never figured out why.

Yours. First thing is check the master cylinder to make sure you have just a little bit of free play. Next crack a bleeder to see if you get anything out. I've had brake hoses collapse so badly I couldn't get a drip out pushing on the pedal. Came on not overnight, but pretty suddenly on a daily driver. Collapsed with fluid in and got hot would expand the fluid and push on the caliper piston? Maybe? ABS? Not sure how, but maybe?

[Aldo525]

Seems to be a common issue in e28's over the years. This is from BMW Car Club forum long time ago, MC was the culprit.

https://www.bmwcca.org/forum/index.php? ... ease.9140/

Others old posts suggest maybe the front calipers pins are not running free cause a bad/dirty/hard pin rubber guides (among others like bad lines and/or hoses)

[austin8753]

i've ran into this issue a few times over the years. the most likely culprit, is a failed master cylinder. make sure you rebuild your hydroboost unit when it's apart, or you'll have a nice ATF leak once it's all done. ask me how i know...

the 750i master cylinder is a popular upgrade and significantly cheaper than the e28 535i part. would recommend going that route, and i'd avoid the APA/URO Parts master cylinder, if possible.

[stuartinmn]

Assuming you mean an e32 750iL, I looked at realoem to get an idea of the price (I realize the numbers there aren't necessarily real world, but should work for comparing the relative pricing.) The e28 master cylinder is priced at $404.76 while the e32 master cylinder is priced at $745.64 and is shown as not available?

[austin8753]

Assuming you mean an e32 750iL, I looked at realoem to get an idea of the price (I realize the numbers there aren't necessarily real world, but should work for comparing the relative pricing.) The e28 master cylinder is priced at $404.76 while the e32 master cylinder is priced at $745.64 and is shown as not available?

yes, sir, e32 750i/il. dealer list prices for parts are absolutely insane, but doing real world shopping (aka, not buying parts at the dealer), my experience showed that the 750il m/cyl was cheaper than the e28 535i m/cyl.

from one of my local vendors, at this time:
current price for e28 535i w/ ABS: 34 31 1 155 270, $335.18
current price for e32 750il: 34 31 1 156 643, $197.39

both are ATE units.

[stuartinmn]

Thanks. I'll call Blunttech on Monday to check their current pricing. The 750 master will work okay with the stock rotors and calipers?

For what it's worth, after sitting overnight the brakes have freed up. I haven't tried driving it yet, but I suppose I should take it for a short trip around the neighborhood to see if it acts up again.

[austin8753]

Thanks. I'll call Blunttech on Monday to check their current pricing. The 750 master will work okay with the stock rotors and calipers?

For what it's worth, after sitting overnight the brakes have freed up. I haven't tried driving it yet, but I suppose I should take it for a short trip around the neighborhood to see if it acts up again.

yes, i've done it several times. you will notice a slightly stiffer pedal due to the increased diameter of the piston on the 750i m/cyl.

the best way to diagnose that issue for certain: start the car and go for a short drive. if the brakes work as normal when the car has been sitting/car is cold, but start gradually applying as the car warms up, you've got a bad m/cyl - at least, that's what i've found. best of luck!

[stuartinmn]

I just checked Blunttech's website. They list two ATE masters for a 535, one for $234 and one for $321 but no indication what the difference is between the two. For the 750 they also list two masters, an ATE for $232 and a URO for $155. The part number for this ATE master is different yet from the two listed for a 535 (they use ATE part numbers, not factory BMW part numbers, so I don't know how they compare to the numbers you posted). Curious.

[austin8753]

I just checked Blunttech's website. They list two ATE masters for a 535, one for $234 and one for $321 but no indication what the difference is between the two. For the 750 they also list two masters, am ATE for $232 and a URO for $155. The part number for this ATE master is different yet from the two listed for a 535 (they use ATE part numbers, not factory BMW part numbers, so I don't know how they compare to the numbers you posted). Curious.

the e28 has two types of master cylinders, one for cars equipped with ABS, and one for cars without.

750i master cyl has a BMW part number of: 34 31 1 156 643, or ATE part number of: 010210.

[stuartinmn]

To close out this discussion, it turned out the problem was actually failed seals in the hydroboost brake servo - apparently it wasn't releasing pressure on the master cylinder. Of course, it figures that it's a lot more expensive part to replace than the master.

[Aldo525]

To close out this discussion, it turned out the problem was actually failed seals in the hydroboost brake servo - apparently it wasn't releasing pressure on the master cylinder. Of course, it figures that it's a lot more expensive part to replace than the master.

Some people say the Hydroboost can't be repaired just replaced by a new one.....but it's worth trying this before buying a new one:

https://e28goodies.com/product/hydroboo ... build-kit/

[stuartinmn]

Too late for that now.

[Mike W.]

To close out this discussion, it turned out the problem was actually failed seals in the hydroboost brake servo - apparently it wasn't releasing pressure on the master cylinder. Of course, it figures that it's a lot more expensive part to replace than the master.

Some people say the Hydroboost can't be repaired just replaced by a new one.....but it's worth trying this before buying a new one:

https://e28goodies.com/product/hydroboo ... build-kit/

The servo or the regulator? I've never been inside the regulator, but the servo is usually fixed with just the one big O ring. I forget who now, might have been Jay, but someone had kits for the servos with all the seals, there's about 6 or 8 total, but usually they just leak and need the one big O ring.

[stuartinmn]

It was the servo itself. When the master cylinder was removed, power steering fluid gushed out of the servo. (It also leaked at the other end inside the passenger compartment, so the carpet got soaked. )

[Federico]

I've had this problem myself. The rubber orings in the control piston start leaking into the purple chamber shown below, and this pushes the servo piston/bobbin as if you were riding the brakes.





IIRC, the orings are common metric sizes.

[Mike W.]

It was the servo itself. When the master cylinder was removed, power steering fluid gushed out of the servo. (It also leaked at the other end inside the passenger compartment, so the carpet got soaked. )

Gushing out of the servo when the big O ring is leaking is common, no big deal. Soaking inside the passenger compartment, arrg, big deal.

I've had this problem myself. The rubber orings in the control piston start leaking into the purple chamber shown below, and this pushes the servo piston/bobbin as if you were riding the brakes.





IIRC, the orings are common metric sizes.

Federico, great illustration, I've never seen a blowup anything like that!

[Federico]

Federico, great illustration, I've never seen a blowup anything like that!

The last picture is from this site: https://www.shiftbmw.com/model/e32/brea ... e-booster/

[Maaaate]

Posted on this before - worth checking the master first - but this excellent link solved the problem, very inexpensively, for me.
https://bigcoupe.com/phpBB3/viewtopic.p ... ing+brakes
BTW, when I had the master out, for no apparent reason all the hydro fluid drained out of the booster after a couple of days, but when I put the master back on and refilled, there was no problem.