Mystery : 1 Thunk from Brakes When Almost Stopped

[bruce.g.cameron]

Hi Peach Parts

I've been mystified by a braking phenomena I haven't been able to solve on my 1989 190D with 96k miles.

When I am slowing down, about 2-4 feet before the final stop (i.e. going very slow), I get a single "thunk". I feel it through the brake pedal, and increasingly I can hear it.

When I get the pulse through the brake pedal, it sort of endures a little - not instantaneous, but sort of feels like there was a dirty spot on the disc that I ride through. However, discs are quite clean, and I only get this thunk once per slow down.

I had the wheels off recently to inspect - all four rotors look good, and seemed I had plenty of pad on all corners. Front rotors pads and rebuilt calipers done 6k miles ago, rears done 30k miles ago.


I have a couple of thoughts, but have not been able to figure it out:

- Could this be something in the Master Cylinder?
- Could this be one of the Calipers - like a rust spot in the bore?
- Could this be a loose control arm or sway bar?
- I haven't gone back to check that all 8 caliper bolts are torqued. Could this be a caliper rotating about one of the bolts?

[Rogviler]

When I had a similar thing happening it was loose caliper bolts. Not finger loose, just enough to let the caliper move. I'd double check those and if any are loose make sure you put thread locker on before tightening them down again.

-Rog

[Diseasel300]

x2 on the caliper. I had this issue in my Honda and it got to the point of being extremely disconcerting. You'd almost stop and *BANG*. You could both hear and feel it and it was clear it was from the front of the car. Was a caliper bolt was loose.

[rocky raccoon]

Fix this right away. I had a caliper bolt on my '83 W123 completely fall out and I did not know it until the caliper rotated and locked a front wheel while braking at speed on an interstate.

[bruce.g.cameron]

Thanks - will report back

[optimusprime]

Brake material on the disc pad, will have come away from the metal backing plate .And as you put the brake on it moves till it can go no more then you get the thud

[bruce.g.cameron]

Here's my update so far.

- I checked, then loosened and re-tightened the caliper bolts on all four corners. I could not find a loose bolt.

- I have not yet checked for brake pad material that has separated from the brake pad backing plate.

- I found play in the driver front caliper, enough just to rock it a little, and noticeably different from the passenger front. When I took the floating half off, I found a little play in the integral sleeve. I took a video below.

https://www.youtube.com/watch?v=oTRUk--PLJA

I have a hard time thinking this is the source of it somehow, didn't seem like enough play. Anyone have experience with it?

[torsionbar]

The part you're wiggling in the video is the caliper sliders. A little play in those is normal and in fact they should move smoothly and freely in-out.

Did you check the caliper carrier bolts? The carrier is the part that those sliders insert into, and have two large bolts on the back side attaching it to the wheel hub.

[optimusprime]

check the disc pad ,material is not fixed on . They do come away on the cheaper disc pads that you can buy .

[tangofox007]

Originally Posted by bruce.g.cameron

When I am slowing down, about 2-4 feet before the final stop (i.e. going very slow), I get a single "thunk".

I had a bad lower ball joint causing a clunk just as I came to a stop.

[bruce.g.cameron]

Thanks for the ongoing suggestions!

I have checked the caliper carrier bolts - that was my "4 corners" bullet above.

Will pull pads on all four corners next to see optimusprime's hypothesis.

I will then check ball joint and control arm bushings. I doubt it is either : I replaced them within last 6 months / 3k miles.

[kuene]

Prob stupid question, but I had a clunk coming from some where under the car just before a stop. Couldn’t for the life of me figure it out, I was checking my suspension, drive shaft, etc. turns out my vac line to trans got disconnected. It was just a clunky final downshift. Amazing how disconnected vac line didn’t seem to impact shifting all that much otherwise.

[bruce.g.cameron]

OK, I have some test results.

- It is not the caliper bolts.
- It is not a brake pad. I remove all of them and checked.
- I shook the (new) ball joints, and don't believe this is the source.

I'm not sure what that leaves me
- Could a bad sway bar bushing cause this?
- Trans vac line?

[kuene]

Check leaky trans vac line

[Junkman]

Does this car have a guide rod and guide rod mount? I would get 1 clunk when braking on my SD and that was the problem.