Right rear brake dragging LONG
 

Hi,

I have a brake problem with the right rear brake.
I'll try to give as much info as I can.
The pads drag on the rotor causing to make it extremely hot, i can smell the brake when I'm stopped at a traffic light etc.
The brake doesn't always drag. It occurs at about 50 % of the drives I take.
it doesn't occur after hard or soft braking,there is no type of braking that seemes to bring it on.
The brake pedal is very high so not much free play.
When I aply the brakes they seem to work perfectly the pedal feels hard.
I've recently replaced the pads because i thought the problem could be related to this,no change but I noticed with replacing that I couldn't push the pistons back into the callipers.I had to bleed the calliper to push the pistons in.While bleeding the pistons could be pushed in easely and without and rough spots.
The rotor looks in good shape,not warped just a little bit grooved.
Could it be that the brakepedal doesn't have enough freeplay so that maybe the brakefluid can't bleed back to where ever it has to go back to ?
Any help greately apreciated,and please be as detailed as you can on how to fix this.
Sorry for my bad english too. I'm in the Netherlands

/8 200,

This is a relatively frequent problem. There are a number of theories in this Forum about it, ranging from the rubber hoses that jump from the steel brake lines to the calipers going soft and somehow sealing the pressure in the caliper, to the master cylinder somehow not releasing the pressure when you take your foot off the pedal.

I personally do not believe the hose theory as the amount of fluid that needs to move when you step on the brakes is very small, it only makes up for a less than a mm travel of the pads, the swelling of the hoses due to the increased pressure and the flexing of the caliper as the disc is squeezed. By design the hydraulic system is not supposed to push fluid around as otherwise the brakes would react at different times and unequally, depending on the length of the flow path, etc.

I suppose it is possible your brake pedal mechanical linkage could be keeping some load on the master cylinder and that could cause the problem.

My Mercedes manual tells me it is the caliper piston seal. Under normal operation the piston seal, a square cross section rubber ring that sits in a recess in the caliper cylinder bore, deforms and stretches when the piston moves to squeeze the disc as you press the brake pedal. When you let your foot off the pedal, the source of pressure is released and the lines shrink back to normal diameter, the caliper "C" shape is restored to its rest position, and the rubber seal relaxes, pulling the caliper piston and pad back away a fraction of a mm from the disc.

When the seal wears, it loses its grip on the piston and then there is nothing to pull the piston and pad back from the disc. Consequently the pad stays in contact with the disc and the disc heats up. As the heat penetrates the caliper the brake fluid can heat up, and swell, and the process of imitating the application of pressure to the brake pedal starts and you get more drag, then more heat and so on.

The fix is to have new calipers installed from a reputable supplier. It is relatively frequent that the rebuilt calipers do not have properly matched pistons and caliper cylinder bores so the rubber seals do not start with the design pre-load and you can have the same problem again. The M-B manual has a test that measures the pull back of the rubber seal, and describes the symptoms of not having the pull-back, then tells you to rebuild or replace the calipers.

Hope this helps. Jim