View Full Version : brakes grabbing and pulsing

03-10-2005, 06:35 AM
My brakes are "grabbing" at low speed; this is particularly noticeable when parking or inching up to a stop light. When completely stopped at a traffic light, there is a "pulse" felt (and heard) at the pedal. It is a click, like an old clock every couple of seconds. Any ideas?

03-10-2005, 08:02 AM
If you mean that you feel pulsing in the brake pedal when you are at a stop light, you can also feel this pulsing in the steering wheel, your brake bomb is at the end of its life, replace as soon as possible.If you feel pulsing when you apply the brakes before you come to a stop, then it could be warped rotors or control arm bushings.
Pulsating brakes at stop on 750iL
I had the same pulse in brake pedal when I stopped. It progressed to pulsing the steering wheel when turning also... it would kick about one inch back and forth.... All I did was change the brake bomb and all is perfect ever since.
I was told by my local BMW dealer that I needed a new brake booster at a cost of 1100 + labor to fix the slight pulsatingof the brake at a full stop. I decided to buy the same ATE part myself for 345 and had a master cylinder thrown in for good measure. The other day I had an independent shop install the parts. It stops slightly better but there is still the issue of the brake pulsation. Also, when I "panic stop", there is a half-second hesitation before the brakes want to grab. My question is, what else could my problems be the cause of. Could it be the "bomb" that I have read about. I have not been able to locate one on my 1990 (07/90) model. Please help.
Delay = Bomb | Brake Pulse = Petosin Filter
I have a 91 750iL and had both of these issues.
Bomb cured the delay (I found that the slight delay causes a driver to hold on tighter to the seat with his cheeks!)
Replacing the petosin filter fixed the pulse on brake pedal. I am making the assumption you are referencing at a complete stop you can feel (maybe hear) a pulse. If you mean "as you come to a stop" then it is probably warped brake rotor(s).
Pulsating brakes at stop on 750iL
The "bomb" is located in the front driver side wheel well just behind the bumper and next to the ABS unit, I found it easier to disconnect the line fitting under the oil filter canister (leaving the main lines attached to the bomb) then pull the unit out the bottom and attach them to the new one before fitting it back in, eliminator the need to pull the front bumper..
The filter you're looking for sits at the very bottom of the power steering reservoir (see#15 in pic below). Once you unscrew the lid off the reservoir...start unscrewing and lifting everything up and off the bolt (the bolt doesn't move). I also suggest using a turkey baster (take yours/your wife's/your sig/other's then replace with new) and a clean empty 2 liter bottle. I say a 2 liter bottle cause that's about the amount the reservoir holds...plus you will know how much (new)Pentosin fluid to replace. If you give your mfg date(located on the driver door jamb) and model...we can look up the filter cartridge number you need to order...there's a couple of different ones depending on yr/model...also you can do a search for "Pentosin" and see if one of the posts contains a list of vendors that sell the stuff other than your local BMW/MercedesBenz/Volkswagen dealerships (it's been posted several times)...and you need to purchase the correct fluid number (88-9/91=CHF 7.1; 10/91-94= CHF 11S)...it should be engraved on the reservoir's lid or on a sticker near the reservoir. DO NOT put ATF in a system that requires Pentosin.
also had front brake vibration/thumping (thumping cured by thrust arm bushing). Then I just had pulsing. I looked at the rotors and determined that they were not the problem. I was directed to the hydraulic pressure regulator and brake "bomb". Depending on your year you either have vacuum or hydraulic brakes. In any case, if you have hydraulic brake assist, the pressure is regulated by the pressure regulator unit which has an attached accumulator (hydraulic capacitor which is round like a bomb) which builds-up the pressure and acts as a pressure reservoir. As I understand things if this is "broken" you will feel the pulsation of the hydraulic pump as pulsating brakes. If you turn off your car, remove the hydraulic reservoir lid and check the level, then pump the brakes, you should see the level rise in the reservoir. If not, then you have a bad "bomb".

I am sure that someone else can correct my description where it is wrong.

I did this test and determining that the hydraulic pressure was not the problem replaced my rotors and the problem went away.

pulsing/ticking sound - petosin filter fix

I think I had the same problem with regards to the ticking/pulsing. And I have solved it.
There is a filter in the bottom of the Pentosin/hydraulic system reservoir.
It seems the old filter had been limiting the flow rate of the system.
Since it has been replaced the ticking/pulsing is gone.
Start with the filter, that costs only about 25 $.
"Da Brake Bomb" is a bit more expensive. See online vendors.
They are also available used, but for this purpose and for reasons of safety I would order a new one.

03-10-2005, 12:22 PM
Great info, thanks!

03-11-2005, 02:27 PM
So I can fix the pulsing with a new fliter. Do you think it will help the "grabbing" of the brakes at slow speeds?

03-11-2005, 02:52 PM
I would change the filter first and then see how it is.
If it does not improve, then we will have to think about next steps.

03-11-2005, 03:29 PM
Thanks. Will do. Anyone have the part number? If I cannot find a new one, I will soak/clean it with some kind of solvent and then reinstall it.

03-12-2005, 12:09 AM
Used my VIN number 3055570 on the online ETK. Better check once more with yours, as I do not know yours (forgot) and do not know if ASC and all that stuff is in your car. Anyway, should be number 15

03-12-2005, 09:30 PM
No ASC stuff. Thanks for the link.

03-14-2005, 10:44 PM
so if that happens, that is normal. Just for info. Pic is from billr aka Bimmernut

03-15-2005, 07:23 PM
I never got the Pic from billr aka Bimmernut?

03-15-2005, 08:43 PM
Hi whit,
small misunderstanding. I mean the above scanned information (not a pic).
Just for info.

03-15-2005, 09:45 PM

Hi whit,
small misunderstanding. I mean the above scanned information (not a pic).
Just for info.