Master cylinder making intermitent clunking noise - United Bimmer Community

**k2bumper** · 03-27-2009, 12:42 PM

I'm having an intermittent problem (every 3 times I drive for a long time) that a slight clunking noise is heard and felt though the brake pedal every 3 seconds or so when my car is warm. It sounds similar to clicking your tongue against the bottom of your mouth. I opened the hood and could feel the clunking noise when putting my hand on the master cylinder.

The braking performance hasn't been impaired at all. It's just a bit embarrassing. I checked the brake fluid and the level looks fine.

Any ideas what could this be? Is it a vacuum leak? Air in the lines? Master cylinder on its way out?

Big Evil · 03-27-2009, 05:28 PM

Sounds like your ABS solenoids. Do you get an ABS light on the dash ever? Maybe run it by someplace and see if they can pull a code.

**3050rpm** · 03-28-2009, 08:55 AM

Just to support Big Evil's suggestion, I get the same sound, and the ABS light is on. I had my shop run the diagnostics, and it came back with a fault in one of the front wheel speed sensors, I think mechanical instead of electronic. I've heard the matter may be fairly simple to deal with, just unplug and clean out the connection and see if that does it. If not, then replace the thing.

**k2bumper** · 03-29-2009, 05:24 PM

You guys might be right... It does seem like a solenoid pulsing-feeling. The only thing that keeps me from being comletely on board is that the ABS light doesn't come on. I think the sensors and lights are working right since when I turn the ignition on, the light goes on and off to show that the light is working and then I do hear the abs solenoids do their thing the first time I go past 3 mph or so. The only thing is that the mysterious sound doesn't sound like buzzing solenoids, like when ABS is active. It's a more rythmic thud, 3 seconds, thud, etc.

I'll ask my mechanic to read the codes next time i'm in. I just worry that since it isn't very reproducible that I might not get a good diagnosis if the codes don't show up with anything.

**3050rpm** · 03-30-2009, 01:10 AM

Yeah, strange situation you have there, with the ABS light behaving normallly, I don't know how to explain that. Next step would be to have your guy run the diagnostics, they just might reveal something definite.

Big Evil · 03-30-2009, 07:52 PM

The ABS systems in the older cars (like ours, pre OBD II) run a diagnostic every time you start the vehicle. If it starts AFTER you start moving the car, then its a wheel speed sensor, vehicle speed sensor, or transmission speed input/output sensor. The great news is not all of them store abs code info so you may not get one OR a light. Just have to have your mechanic follow the directions he can get out of alldata (he SHOULD know what that is). They can be a grand beyotch.

**k2bumper** · 03-31-2009, 07:51 AM

Thanks again for the replies. Is it possible for air bubbles to be in the system and circulating? It almost feels like air bubbles pushed through a tiny orifice. Kind of like how air bubbles can circulate through the cooling system.

**3050rpm** · 03-31-2009, 09:56 AM

Right, the same sensation comes to mind, but I don't think it'd be air bubbles, that'd be indicative of more serious brake system problems. Nonetheless, you've checked your brake fluid reservoir? Noticed anything leaking where the car has been parked?

Big Evil · 03-31-2009, 04:41 PM

Two things come to mind, air bubbles WILL form if you're running DOT 5 brake fluid or some synthetics (its called cavitation). ABS MUST use DOT 3 or 4 (per OEM) to avoid this. But when 3050 mentioned the leak.......I had an issue. My slave cylinder for the clutch had a pin hole leak, and since the slave and master cylinder on my 325is share a common reservoir, guess what (yup, low fluid). And it was the tinniest of leaks too, barely detectable as it would drip out the weep hole under the bell housing while driving. If yours is manual I'd certainly look there. Other option have the brake fluid changed. Most shops have a power bleeder that will suck all the old fluid out the bottom while dumping new stuff in the top and bleeding the system at the same time......the last little kicker, most OEM's actually recommend that service every couple of years. Of course if its due you might as well get the brake pads replaced at the same time.