Immediate if available - rear brakes locked up-smoking! - PeachParts Mercedes-Benz Forum

willrev · #1 03-27-2006, 09:36 AM

Just dropped the toddler at pre school. Car was sluggish. Smelled brakes burning. Both rear sides smoking!

parked car and borrowed car to come home. Recently have replaced calipers on front, all flex brake hoses, pads front and rear. Replaced all rotors and rear pkng brake shoes.

I applied and released the pkng brake. No change. Put car in drive and was not moving.

I am guessing rear calipers have failed. Why now and why both at same time? Help!!!

t walgamuth · #2 03-27-2006, 09:48 AM

seems unlikely that they are the prob.

check park brake ...though it prob wouldnt stop car from moving.

then it seems like an overpressure thing on the rear circuit.

what kindof fluid you using?

tom w

jcyuhn · #3 03-27-2006, 10:02 AM

It's the master cylinder. Known failure mode for MBs from that era. The piston inside the MC which activates the rear brakes sticks in place. It's especially common after bleeding the brakes by pumping the pedal. Only cure is to replace or rebuild the MC. BTDT on my 123 many years ago.

- JimY

andmoon · #4 03-27-2006, 10:03 AM

I tried bleeding the brakes on a 84 500 sel using the pump method...I lost a lot of my braking. I think the MC has multi curcuits. Maybe the rear ones are under pressure.

willrev · #5 03-27-2006, 10:08 AM

I was guessing Master Cylinder. It has two pistons. I am using ATE Fluid.
Otherwise, why would both lock up??

thorsen · #6 03-27-2006, 10:13 AM

Open the bleeder valves on the rear and see if fluid is expelled under pressure without any pressure on the pedal.

andmoon · #7 03-27-2006, 10:16 AM

Since we are dealing w/ brakes...take this w/ lots of CAUTION!

I fixed the brake bleed goof up by applying pressure to a bleed nipple on a rear brake.
I figured the piston had moved farther than it had in years. My intention was to free it for easier rebuild but never got around to it as the brakes worked fine after that.

Stevo · #8 03-27-2006, 10:25 AM

Quote:

Originally Posted by jcyuhn

It's the master cylinder. Known failure mode for MBs from that era. The piston inside the MC which activates the rear brakes sticks in place. It's especially common after bleeding the brakes by pumping the pedal. Only cure is to replace or rebuild the MC. BTDT on my 123 many years ago.

- JimY

Thats interesting..I'll file this away for future reference, thanks...Another reason to have a pressure bleeder.

Ralph69220d · #9 03-27-2006, 12:26 PM

In my opinion, by replacing the rear pads, but not the rear calipers, the caliper pistons are trying to travel places they have not travelled in a long time and are locked up by corrosive by-products.

mplafleur · #10 03-27-2006, 12:52 PM

My money is on the master cylinder. My '89 190E did the same thing. I would have to releive pressure by cracking open the line at the outlet of the master cylinder. For me, I believe it was the front brakes. The calipers had nothing to do with it.

A new MC fixed it. ~$90 I recall.

willrev · #11 03-28-2006, 06:08 AM

Well, I tried to get under it in the parking lot and to no reslove. I was going to loosen the bleeders and try driving it home. We drove it a little before putting on the rollback, but the rear brakes were still dragging.

I picked up a new master cylinder and will let my mechanic bench bleed it and install.

Hope this fixes it.

willrev · #12 03-29-2006, 01:51 PM

Finished this repair today. The resevoir on top of the master cylinder had a blackish sludge floating in the bottom with particulate matter. Removed, cleaned and let dry.

Installed new ATE master cylinder. A whopping difference. My pedal is much stiffer than it has ever been since buying the car. I can tell now that my rear calipers must have been dragging for quite a while - it just got progressively worse.

Wonder what this may do to my fuel mileage???