84 300SD - Brakes Dragging Lightly - PeachParts Mercedes-Benz Forum

Gregory · #1 05-09-2015, 04:13 PM

For a couple months now, I can hear the slight dragging sound of pads dragging on the rotors.

A couple days ago I replaced the rear rotors and pads, they were due. I also cleaned and lubed the rear calipers and hardware. The front brakes and rotors were replaced about 6 months ago.

The dragging noise remains. My friend stuck his head out the window and confirms the noise is coming from the rear, not the front.

If I hit the brakes sharply for an instant the noise stops, until after I use the brakes normally to come to a stop. Then when going again the dragging noise returns, until sharply tapping the brakes again.

Something is causing the calipers to not properly retract after normal braking. Any ideas?

BenzBUSY · #2 05-09-2015, 04:24 PM

Some MB people here might think you are.....imagining odd noises....But in fact...no you are not! You have suggested here that the brake hydrolic system and all off its interconnecting unions...are as correct, from factory spec delivery.....My contention...is that over time, some or most of this hydrolic brake system has failed....

BenzBUSY · #3 05-09-2015, 04:30 PM

Gregory...breaks drag after replacement....usualy because of either ignorance to 'bleed' or even more 'stupid'.....Please...Tell "Charles' even though he has brains.....He's better off....letting morons fix!

Graham · #4 05-09-2015, 05:47 PM

As I understand it, there is nothing obvious in the brake design that retracts the pads. They do run very close to the rotors. What pulls the piston back, is the lip seal on the caliper. If those caliper seals are not in good shape, they won't retract.

This article might help:

Brake Calipers

Here is an excerpt:

Quote:

When the brake pedal is released, hydraulic pressure drops and the calipers loosen their grip. Square cut seals around the caliper pistons deform when the pistons move, so when pressure is released the seals want to return back to their original shape. This helps retract the pistons slightly,allowing the pads to kick back away from the rotors. This eliminates the need for return springs that are necessary in drum brakes.

Not having the heat shield/dust cover on the seals can cause them to fail early.

Skid Row Joe · #5 05-09-2015, 06:57 PM

Check your parking brake pads, linkage, etc.

leathermang · #6 05-09-2015, 09:25 PM

Sometimes the thickness of the new pad is slightly too thick... I have had to sand down some in the past.... hard to keep the surface correct with the back of the pad when that happens....
I do not think that disc brakes ' retract ' at all... they are slightly touching.. or they are squeezing...... this is what makes them self cleaning.....
Touching is not bad... but if they are in contact to the extent that they heat the rotor during regular driving.... which can be determined by checking their temperature after a drive.... usually can be felt without touching the back wheel if you have a problem...
Now there is a situation which I and others have experienced and noted in the archives... where a rubber brake hose de laminates on the inside... a flap .. which can allow the brake to be engaged.... but will not let the brake release.....but that is extreme and you would have more symptoms than you have described... Real Heat.. perhaps pulling to one side.... etc...

DDAY · #7 05-09-2015, 10:46 PM

I had a '83 300 SD with worn out brakes all the way around. The Indy installed new rotors and pads. Less than a year later the "new" brakes were all worn out. The old brake hoses had swollen shut on the inside "called hour-glassing". The high pressure from the master cylinder could force the fluid through the restricted hoses and the calipers could apply the brakes. The fluid could not completely return to the master cylinder thus the brakes were on lightly (and evenly) all the time. He replaced the brake hoses, rotors, and pads. Problem solved.

Ceristimo · #8 05-09-2015, 10:57 PM

There's nothing on a brake system to retract the pads. They push themselves back, and are in fact always in contact with the rotor.
Of course, the drag is so little that no noise or heat should be created.

One thing that might have been overlooked: old brake lines. These can get soft and expand a little, causing the system to keep pressure and brakes to drag.
Rust on the piston would cause this too, but you said you cleaned the brakes - and you'd have noticed the rust on the piston - so let's assume there is none.

EDIT:
Ugh, too late. Again! DDAY right above me already mentioned the break hoses. Well then, that is 2 votes for the brake hoses. Mine is from personal experience as well, since I had the exact same thing happen with the front brakes on my daily driver Mazda.
Of course, parts for that car aren't exactly super expensive, and since I had been struggling with brakes sticking since I bought the car (second hand), I decided to change everything.
Took me two days, but for about 600 bucks it has new soft lines, 4 new rotors, 3 new calipers, new pads all around and new brake fluid. Of course, 600 bucks on a Benz will get you one (1) caliper...

leathermang · #9 05-09-2015, 11:09 PM

Gregory, why don't you put your friend in the trunk and take him for a ride.. perhaps he can tell if it only on just one side... LOL
While the timing of the on and off sound does not really seem to relate to the parking brakes... it would be interesting if you pulled the emergency brake handle while going really slow and it Making the Noise at that time..... see if that affects the sound at all....keep your finger on the button ... just apply it and let it off.... let us know if it changes the sound...

leathermang · #10 05-09-2015, 11:13 PM

Quote:

Originally Posted by Ceristimo

......
EDIT:
Ugh, too late. Again! DDAY right above me already mentioned the break hoses. Well then, that is 2 votes for the brake hoses......

That is Three Votes since I mentioned it two and a half hours before you posted......

Skid Row Joe · #11 05-09-2015, 11:18 PM

Quote:

Originally Posted by DDAY

I had a '83 300 SD with worn out brakes all the way around. The Indy installed new rotors and pads. Less than a year later the "new" brakes were all worn out. The old brake hoses had swollen shut on the inside "called hour-glassing". The high pressure from the master cylinder could force the fluid through the restricted hoses and the calipers could apply the brakes. The fluid could not completely return to the master cylinder thus the brakes were on lightly (and evenly) all the time. He replaced the brake hoses, rotors, and pads. Problem solved.

Proof positive........

As these old cars age, you've got to be ever more vigilant on ALL related parts.

Skid Row Joe · #12 05-09-2015, 11:23 PM

You may end up needing new caliper assemblies, before it's over...... The pistons aren't good forever, ya know.

barry12345 · #13 05-10-2015, 11:35 AM

Your description leads me to think that the return port function of the master cylinder may be getting partially blocked.Deterioration of the rubber in the bore can do this.

If the noise is constant. I would drift to a stop without brake application. Loosen a rear brake bleeder fitting or the line at the master cylinder feeding the rear brakes.Just enough to release a very small amount of brake fluid. If this stops the dragging sound it may be the master cylinder.

Kind of an obscure problem but still possible. When you punch the brakes hard and the noise is not there led me to suspect a reverse sharper than normal pressure wave of the fluid might be clearing the rear brake return port in the master cylinder.

This suspicion could be further verified if you could establish the noise was coming from both rear wheels. Perhaps someone could verify by listening out one rear side window then the other.

This is just one probability though to keep in mind or I would. If it goes this far remember to test before determining the master cylinder is faulty.

Basically how the master works. As the pedal is pushed the rubber seal inside the master pushes past the outlet port. Pressure builds. You remove the pedal pressure the seal is supposed to come back and totally clear the port. This allows all pressure relief. There have been instances where this does not happen properly leaving some residual pressure in the lines.

That is why I would do the bleed test. Much of the need is dependant on if it is both back brakes that are noisey. Only one pretty much clears the master.

leathermang · #14 05-10-2015, 11:55 AM

If fluid is allowed out of the bleeder at the caliper .... the system will need to be bled afterwards.....as the only way for fluid to drip out is for air to replace it going the other direction....

Graham · #15 05-10-2015, 11:55 AM

Quote:

Originally Posted by Skid Row Joe

You may end up needing new caliper assemblies, before it's over...... The pistons aren't good forever, ya know.

I would at least look at the caliper seals. If they have deteriorated, the lip seal won't retract the piston. Had that happen, and new caliper seals solved problem.

Also had one rear hose collapse internally once, but that caused total lock up of brake. I suppose lesser restriction might cause dragging as suggested upthread.

Recently replaced hoses (and corroded tubing) all round. Also have new (or rebuilt) calipers all round. Stil on original MC. (1985 300D)