Rear brakes dragging - PeachParts Mercedes-Benz Forum

kdelacy · #1 08-08-2009, 09:32 PM

Currently having a situation where the rear brakes intermitently start to drag on my 85 300CD.
This started a couple weeks ago, and I immediately though it was a parking brake issue. One night though after a particularily bad episode both of the rotor discs were glowing red hot! Because the parking brake is a drum style setup located in the center of the rotor I changed my mind to dragging calipers. Since then I've replaced both rear calipers, rotors, flexable hoses, and pads. Now they're starting to drag again. I'm guessing there is a sort of proportioning valve somewhere in the hydralic brake line that is malfunctioning.
And thoughts on this would be greatly appreciated!
Thanks,
Ken

Brian Carlton · #2 08-08-2009, 09:51 PM

With all of the new hardware, it appears that you've covered all your bases at the wheels. This leaves the m/c as the likely culprit.

When the problem occurs, crack the line at the m/c to relieve any pressure to the rear brake system. On the W126, the forward line on the m/c goes to the rear brakes. The W123 is likely the same, but I'd appreciate confirmation from someone who's got one.

If the problem goes away (before you hit the brakes again).........the m/c is the culprit.

kdelacy · #3 08-12-2009, 09:57 AM

Thanks Brian will keep a wrench handy for the next occurance, and let you know how things work out.
Ken

leathermang · #4 08-12-2009, 12:02 PM

I am going to mention something not likely the case for this car...with its independent suspension ....which means two rubber brake hoses on the rear...
but if this were a live axle auto then it is possible for a bad brake hose to allow this to happen... and since it can not be seen from the outside is very hard to diagnose.
I have had brakes which would not retract due to inside delamination of the rubber layers... allowing flow of fluid one direction but slowing or stopping it from the other...which is needed to allow the brake pads or shoes to let go for normal driving position.
One of our moderators I think had this happen also... Mr. Smith..

Oracle12345 · #5 08-12-2009, 12:06 PM

Is your parking brake out of adjustment?

Brian Carlton · #6 08-12-2009, 12:12 PM

Quote:

Originally Posted by kdelacy

Since then I've replaced both rear calipers, rotors, flexable hoses, and pads.

......

#7 08-12-2009, 01:52 PM

Yes, as Brian pointed out, the problem is at the Master Cylinder. Been there, done that! To confirm this, the next time they hang, use a tubing wrench and loosen the fittings at the master cylinder. If one of them squirts fluid while no one is pressing the brake pedal then you KNOW that it is a hydraulic issue and since you have replaced everything else, then it must be valving in the master cylinder.

Biodiesel300TD · #8 08-12-2009, 03:26 PM

You could also try loosening the parking brake to eliminate that as an issue. You could have broken shoes or something that is getting in the way intermittently.

rrgrassi · #9 08-12-2009, 03:32 PM

Quote:

Originally Posted by Biodiesel300TD

You could also try loosening the parking brake to eliminate that as an issue. You could have broken shoes or something that is getting in the way intermittently.

The chances of both sides of the e-brake system failing is not very likely.

#10 08-12-2009, 03:52 PM

If it were the emergency brake locking up, that would be the area of the rotor that is red. If it is the pads against the rotor, the rotor itself will be red. It will be the swept area that will be red.

whunter · #11 08-12-2009, 04:00 PM

Quote:

Originally Posted by kdelacy

Currently having a situation where the rear brakes intermitently start to drag on my 85 300CD.
This started a couple weeks ago, and I immediately though it was a parking brake issue. One night though after a particularily bad episode both of the rotor discs were glowing red hot! Because the parking brake is a drum style setup located in the center of the rotor I changed my mind to dragging calipers. Since then I've replaced both rear calipers, rotors, flexable hoses, and pads. Now they're starting to drag again. I'm guessing there is a sort of proportioning valve somewhere in the hydralic brake line that is malfunctioning.
And thoughts on this would be greatly appreciated!
Thanks,
Ken

This is a safety issue = can kill you or others!!!

* STOP driving the car.
* Replace the defective brake master cylinder.

Brake:
Brake:

Have a great day.

Oracle12345 · #12 08-12-2009, 04:09 PM

Quote:

Originally Posted by LarryBible

If it were the emergency brake locking up, that would be the area of the rotor that is red. If it is the pads against the rotor, the rotor itself will be red. It will be the swept area that will be red.

the emergency brake doesnt need to be complely locked up to cause dragging. They could be out of adjustment and working completely fine but still be constantly touching the rotor. If this were the case the rotor would be warm. to make them red they really have to be out of adjustment or you are going fast. But it seems if the rotors were red the pads should be melting and fusing the pad and rotor together.

Dragging brakes is ether the master clyinder or out of adjustment emergency brake.

rrgrassi · #13 08-12-2009, 04:18 PM

Since the rotor got red hot, you may want to replace them again, due to possible warping.

Biodiesel300TD · #14 08-12-2009, 04:49 PM

Quote:

Originally Posted by rrgrassi

The chances of both sides of the e-brake system failing is not very likely.

They may not have failed but they could both be dragging. And a simple 5 minute adjustment under the car will tell you wether that is the issue or not. If nothing changes after loosening the parking brake cable. Then you've eliminated the parking brake as an issue.

Mine had been dragging slightly for quite some time. My milage was consistantly around 23mpg. I loosened the cable and it jumped to 26mpg.

#15 08-12-2009, 08:24 PM

Quote:

Originally Posted by Oracle12345

the emergency brake doesnt need to be complely locked up to cause dragging. They could be out of adjustment and working completely fine but still be constantly touching the rotor. If this were the case the rotor would be warm. to make them red they really have to be out of adjustment or you are going fast. But it seems if the rotors were red the pads should be melting and fusing the pad and rotor together.

Dragging brakes is ether the master clyinder or out of adjustment emergency brake.

Look guys, as I said before I've been through all this before. The emergency will not hold enough to cause the rotors to turn red. If they DO, the swept area around the drum portion of the rotor will be red, NOT the pad swept area of the rotor. The emergency brake could indeed make the rotor warm but it can NOT make the pad swept area red.

The problem lies in the hydraulics. The master cylinder can allow a hydraulic lock such that the pressure must be released. You can do this by loosening the line at the m/c or if the system is not tight, it will eventually bleed off.

BTW, after I went through this with my rear rotors being RED, I ran those rotors a few more hundred thousand miles before replacement. If they aren't warped, don't worry about them. These rotors are amazingly tough.