Brakes pop(spongy?) when stopping…sometimes?

[Freight]

Down to what seems to be the last major order of business - occasionally when coming to a complete stop, the pedal will feel like it’s popping or bubbling. Hard to explain, as the pedal otherwise feels fine. It almost feels like what you’d imagine it might feel like if the pads were hitting speed bumps or potholes on the rotor. Pads and rotors were checked and are good. Not sure if that counts as spongy, and I can’t seem to hear or feel it in the way the car stops. This happens only when coming to a near complete stop, under 5-8mph, and doesn’t happen every time. 

With the car off, I can get 15(+/-1) pumps of the pedal before it goes rock solid. 

Brake lines at the calipers all look good, aren’t leaking, or cracking. 

I do have fluid here, which realoem says it a voltage regulator? Hard to tell where exactly it’s weeping from, or if it’s actively weeping at all.
 

there is also light fluid here, at the PS pump, but it’s not from dripping done the hose.

 

Brake fluid and PS levels look good, and I don’t seem to have any peculiarities in the power steering performance.  Also, no drips on the driveway ever. Brake pads and rotors and calipers are good, and perform well during “spirited” driving.

I’d love to keep the current setup rather than converting to a traditional vacuum booster, it at all possible to do so.   Hopefully this is a fairly simple fix?
 

[stuartinmn]

Sounds like an ABS failure to me.

[Freight]

Sounds like an ABS failure to me.

My car is not an ABS car…to my knowledge. No abs light on the cluster, and I don’t think it shows it on the options list with the vin decode

[Aldo525]

Air in the brake lines??...did you bleed the circuit??...that is easy and cheap. Then check the brakes rubber hoses in every corner, then the Master Cylinder and Brake Booster. When pedal is depressed 15 times the ATF level is 1/4 inches to the top??....lastly the "bomb", the baloon inside could be hard or broken

[Freight]

Sounds like an ABS failure to me.

I’ll be damned. I looked in the engine bay and have abs. Confused, I looked back at the POs service binder, and the cluster was replaced(and mileage matched) with a serviced one from a cluster specialist about 15 years ago. Guess they didn’t get an ABS cluster. Went and pulled the plug for the ABS, and the problem is gone. I drove around testing it with the abs still live, and noticed I can hear a hiss/buzz occasionally. When it’s making noise, I have the strange pedal behavior. When it’s not, the pedal is normal. 

With the relay out, the noise is gone, and the brakes function as normal. With it still plugged, I can lock up the brakes when it’s not buzzing. When it’s buzzing, stomping on the brakes in in the rain(lucky weather to figure this out) seems to lock up some wheels and the others get ABS pulses that are inconsistent and slow.

thanks for leading me to the right place!

[gadget73]

When I've had false ABS activations when coming to a stop its usually been rust in the tone rings, or if the gap between the sensor and tone ring is adjustable, having that too large will do it too.  Basically the ABS system loses pulse when the wheel is barely rolling and it triggers the antilock stuff.  I don't know the BMW system specifically but early ABS tends to have a lot of similarities. 

 

[Freight]

When I've had false ABS activations when coming to a stop its usually been rust in the tone rings, or if the gap between the sensor and tone ring is adjustable, having that too large will do it too.  Basically the ABS system loses pulse when the wheel is barely rolling and it triggers the antilock stuff.  I don't know the BMW system specifically but early ABS tends to have a lot of similarities. 


 

I was checking my diff mount yesterday so I was all the way under the car, and saw that the tone rings were covered in rust. I’ll get in there and clean them.