Is there such thing as a slow/lazy caliper

[crdiscoverer]

My car has some minor shimmy under braking, but only when braking hard and from higher speeds, say 70+. Otherwise you can't feel it. Also, if you quickly step on the brake at high speeds, it very momentarily wants to go right but as soon as the brakes bite, it straightens.

Without getting into the "brake rotors don't warp" rabbit hole, I know brake shimmy can be 99% attributed to bad thrust arms leaking fluid but I checked all of them and at least visually they look fine. They are relatively new, the whole suspension is. Tires are wearing even and the car doesn't pull to any side and has no shimmy off the brakes.

I recently replaced pads and rotors and refreshed my calipers with new slider pins, took out the pistons too so I'm suspecting one of the front calipers could be ever so slightly slow in its actuation? I searched but I can't see anyone else saying something similar. Front and rear wheels brake with exactly the same force when tested individually (within margin of error). What I can't see is the braking force curve as you only get the final/total torque they exert.

[Chedley]

Sounds like the brakes do not respond immediately when you step on the brake pedal at high speeds.
When you replaced the pads and rotors, did you inspect and clean the calipers ? Did you bleed / flush / replace the brakes fluid ??

[E39 Newbie]

What you're saying is, indeed, possible. I've had it on other cars. One caliper bites before the other. If I remember, I replaced the calipers on that car (not a BMW) and the issue went away. When you pulled the pistons was there any pitting or scratches or whatever in the bore? What's the condition of your front brake hoses. Maybe one is getting soft?

[Dking078]

Bleed first, then ask questions. You could have air in the lines. Possibly due to a cracked hose from age.

If bleeding checks out buy a thermal IR heat gun, drive around normally, measure temps of the rotors / calipers. Then beat on it and see what happens.

Either your right is sticking or your left is not engaging due to air.

[Mike WW]

I would call it a sticky caliper, rather than slow to respond, but yes, it does happen. Air in the system and bleeding to eliminate it is never a bad thing, and start there, but you might need to just rebuild the calipers with new seals and clean them up. Easy job. Particularly in your location moisture and rust could be a problem.

[ross1]

I would call it a sticky caliper, rather than slow to respond, but yes, it does happen. Air in the system and bleeding to eliminate it is never a bad thing, and start there, but you might need to just rebuild the calipers with new seals and clean them up. Easy job. Particularly in your location moisture and rust could be a problem.

I'd suggest the caliper isn't sliding freely.
Rotors could also have pad material unevenly imbedded. Can you see witness marks of the pad shape?

[dtw]

Yup, is a thing. Slider hanging up or interfering. Start there for easiest option - could be as simple going over the slider with some emery cloth then hitting it with a shot of lube.

Less fun option is corrosion in the piston bore is causing friction.

[Toby B]

Yep,
they can do that. One wheel bearing can have a little more runout,
and that caliper gets knocked back just a little.

Or one caliper''s got more resistance OR more resilience than the other...

I bought inexpensive caliper rebuild kits once, and got that 'feature'-
next time, I'll suck it up and pay the (significant at the time) ATE tax.

t

[BimmrMeUpSnotty]

Bleed first, then ask questions. You could have air in the lines. Possibly due to a cracked hose from age.

If bleeding checks out buy a thermal IR heat gun, drive around normally, measure temps of the rotors / calipers. Then beat on it and see what happens.

Either your right is sticking or your left is not engaging due to air.

That!!!^^^

[ross1]

Yeah, I agree with bleeding first. Followed by examining for free movement of the caliper in the bracket.
Don't neglect to examine the rears as well.
Shimmy and darting to one side could be suspension or alignment but check the rotors for uneven pad material deposits too.

[philly98540]

Had an old ford SUV that would do something similar. During braking it would pull for a brief moment to one side then it would pull straight. It ended up being the front brake hose being so old the inside swelled closed on the one side. The PO changed only one of the front hoses and not both. Grrrr. I put all new hoses all around and the braking was back to normal.

[BimmrMeUpSnotty]

It just cracks me up how some people will only fix the one side that was acting up, what is it? They just completely lack the basics of common sense???

[StephenVA]

all parts get Axle set for all mods, repairs, and anything regarding brakes!
agree-with all on the steps
bleed system again including ABS Cycling
clean and ensure the pads ears are not catching on groves on the brackets (you did file the areas flat?)
remove and replace the caliper boots clean and install DRY the caliper pins

[crdiscoverer]

Okay, thanks. When you look for sticking calipers what you get is calipers that don't let go, which is sort of the opposite, so I didn't want this to sound like another one of those cases.

I should've mentioned a few things in my first post as I have done a pretty major brake overhaul recently:

1. About a couple of years ago the master cylinder got replaced.
2. Air got into the ABS module after that so I've had to purge the whole system, including cycling the ABS module.
3. More recently, I replaced all 6 hoses (wheels + ABS) with OEM ones
4. Replaced the slider pins and the rubber sleeves (didn't grease them as per instructions)
5. Removed the pistons and inspected them
6. Changed pads and rotors (Textar pads, Balo rotors)
7. Bled the system again, also the ABS module and added new ATE fluid
8. Checked tire pressure

After all that, which happened maybe 4 months ago, the car braked perfectly, better than ever before. But I haven't been driving too much so in those 4 months that'd be like 1500 miles at most.

As an alternative to the caliper being bad, it could be some pad material deposited on the left front rotor (since it pulls right). My steering rack also has some slight, unrepairable wear but that wouldn't explain why it was braking straight and shimmy-less before (rack has been like that for years). There are a couple of very minor grooves in one of the front rotors, can feel them but barely. When I inspected the pistons they looked fine. I just cleaned them up and reinstalled them.

Are the piston seals/o-rings suspect? I didn't replace those.

[StephenVA]

The piston boots and O-rings are one time use items. Buy the repair kit and replace the inner seal and dust boot. Many times on install, the inner seal will get pinched and create a "tight" piston.

[crdiscoverer]

Yeah, that could've been it. Sigh... Not looking forward to taking everything apart again. Weird that everything was fine for a while.

[JimLev]

Just behind the boot make sure there isn’t any rust, that will cause the piston to hang up.
Don’t bother trying to sand it off as that will also remove the coating and cause it to rust even more.
New pistons are available, put a little grease on the part of the piston that sticks out of the caliper.

[crdiscoverer]

Thanks, Jim.

First thing I will do is take the car for a wheel alignment. That's probably the only thing I haven't done recently. I don't see any uneven wear but it could've been a recent pothole or something. It's cheap and I think it's due anyway. If that doesn't change anything then I'll start taking things apart again. The piston boots was one of those things that once I had everything disassembled I said "oh s... I forgot about buying this crap" and had to put everything together as it was. We've all been there I suppose.

[Chedley]

I doubt if an alignment will fix the problem.
If it was a mis-alignment, you would see the tilting at all speeds, and not only when applying the brakes.

[crdiscoverer]

I checked everything again and I think I've isolated the problem to a partially stuck ABS module valve/solenoid, very similar to this:

https://www.youtube.com/watch?v=Dh82Ohnl9IE

I see the hydraulic part is non-serviceable so I'll try getting it unstuck with magnets, compressed air and some mild lubricant. If that doesn't work then I'll try a torch. Since it's not completely stuck I think I have good chances of freeing it up. I suspect after the main cylinder replacement, some dirt from the old one found its way there and caused the issue. The good news is that since my car is so old, the ABS electronics are under the dash so the module is mostly mechanical bits.

[crdiscoverer]

In case you have never seen one of those very early, electronic-less modules (also found in E36/E38s). It looks like BMW always thought "hey, that thing is very close to the exhaust, better put those electronics elsewhere" but then around 98/99 they said "hold my warm beer".
The valves in these ones are probably serviceable, but the idea is not to desolder anything. I'm using a strong magnet to hear for those clicks and try to loosen up any dirt that could be lodged in there. I'll probably get a used module if that doesn't work and I'll further tinker with the worst one.

[sienayr]

It may not be the valve or solenoid, but that thin ribbon connections causing bad connections. See here, this is for e36:

http://www.staffordnet.net/repairs/bmw/abs_pump_rewire.htm?i=1

http://raysaywhat.blogspot.com/2014/...8-bmw-e36.html

Not exactly the same design but may be worth checking as a root cause.

[crdiscoverer]

Thanks. From what I've read, these modules are relatively bulletproof. I tested and cleaned the connections and it all seems fine. There is a ground wire in the middle which can be seen in the first pic, the insulation was completely gone so I also fixed that just in case as well as RTV-ing the long gone weathersealing. I am hoping that lightly tapping on the solenoids and the magnet trick had some effect. I will need to go out and drive it. If not then the next step would be to completely remove the module from the car and bench test it using compressed air. I am avoiding that mess until I've exhausted other methods. I can see those modules are very cheap on eBay, probably because they lack the electronics, so I won't waste too much time troubleshooting this one. If I end up replacing it, I will take the bad one apart for sure. There's a surprising lack of information about these early modules. These may even have their own bleeder screws, negating the need for a scanner to cycle the valves. Not sure about that but I'll find out if I can.

The module became the primary suspect after I took both front calipers off again to see if there were some signs of sticking, I bled the system for the n-th time and noticed that the front left corner was bleeding at a slower rate. Unnoticeable unless you are really looking for it. Most people who have stuck solenoids have completely dry lines and rusty rotors. I didn't. The module is basically acting like a swollen rubber brake hose. It all makes a ton of sense now. It still could be something else blocking the flow of fluid downstream, but I'm betting I found the culprit.

[StephenVA]

Congrats!

[SW530]

When my caliper was sticky/lazy I wanted to do a complete brake job but it really didn't need it right now and parts were days away, so I went to NAPA, bought a pin kit (they're serviceable parts that should be replaced at every brake job), the appropriate lube, the good brake fluid, and all new hoses. I ended up going back for new bleed screws because those were messed up as well. So I replaced the pins sets making sure they were properly lubed, inspected the pads and rotors (nothing wrong, just 60% worn), replaced the brake hoses and bleed screws, and bled the system. Power bleeders work great on these cars making bleeding take about as long as it takes you to hit all for corners, hook up the hose, turn the bleeder screws, and move on to the next one.

I might have missed it.. but old brake hoses will cause this problem. They collapse internally. At 20+ years old they're ready to be replaced..