S211 - rear brakes always slightly on - SBC ??

[Mark68]

Hi everyone. I hope you might be able to help me with this. I have an 03 plate 320CDI ( 135K miles ) that has developed an odd problem with the rear brakes.

Basically the rear pads are always slightly in contact with the rear discs, leading to excessive wear, reduced MPG etc : the car always feels like it is being "held back", so I've stopped driving it.

The indy I use for servicing ( since 101K miles ) has not seen the problem before, suggested it could be the SBC pump playing up, but recommended obtaining a second opinion from an MB dealer.

I paid my local MB dealer for a diagnostic check but they had never seen the problem before either, so said they would send a technical report off to MB head office and they would have an answer in 3 or 4 days : three weeks ( and several chaser calls ) later they still haven't got back to me....

Any thoughts as to what the problem might be ?

Thanks in advance,
Mark68.

 

[WG M-B]

Is it happening both sides?

 

[Mark68]

yes both sides. All the components have been greased etc but I still have the problem...

 

[WG M-B]

Well it is very strange. Not something I have come across before! Obviously the dealer would have checked it wasn't the park brake causing the binding and not the braking system? Can the bind be felt by spinning the wheels on a hoist?

 

[Mark68]

The indy tells me that he checked the parking brake and the wheels are difficult to turn when up in the air.
Out of interest, if it was just on one side would you have a suggestion ?
( thanks for the replies by the way )

 

[Mark68]

Hi Wayne.
I'm still struggling to get to the bottom of this problem.
Did you have a suggestion if it was just the one brake pad that was always in contact with the disc ?
( both of my rear pads are, but it still might give me an idea as to what is wrong with mine).
Thanks in advance,
Mark68

 

[SL300-24]

Has anyone checked the caliper pistons for corrosion? I would have thought that would be one of the first checks. You may need to strip and overhaul the caliper(s).

 

[Mark68]

Thanks for the reply.

I believe they have been looked at but will double check.

 

[Dieselman]

We have had this raised on here previously and it was SBC holding the brakes on.

At least the rear disks aren't vented.
years ago Ford Cargo lorries had a problem with sticking rear calipers, but due to the torque the drivers didn't notice until the disk wore through and the pad then jammed the vets solid pulling the lorry off the motorway at speed.

 

[Mark68]

many thanks for the reply. In that case I guess a new SBC pump is the way to go here ?

 

[Mark68]

new SBC pump fitted today - but still the same problem is there : feeling disappointed now !

any other ideas welcome please.......

Thanks in advance,
Mark68

 

[mersum1es]

I've read this happening couple of times. Calipers, sliding pins and pads look ok but (due to bad installation sometimes?), something has been slightly bent, not noticeable but causing jamming. Only cure was change of the all brake parts (calipers, discs, pads)...

 

[RBYCC]

Front and rear pads are always in light contact by design with the rotors.
The caliper has no return spring to pull the pad back off of the rotor.
Unlike the front, it's always difficult to spin the rear wheels due to drivetrain resistance.

You mention "excessive wear"...how was this determined ?
Pad wear?
Rotor wear?
How many miles driven to realize the wear ?
Have you mic'd the rotors to see if the thickness varies?

The only component that could cause drag on the rear would be the parking brake.

SBC was a disaster for Merc....
Was your new SBC pump fitted for free?
In the USA there was a warranty addendum covering the SBC 10 years from initial date of service.

 

[Mark68]

many thanks for the replies.

There is quite a lot of pressure being applied to the rear pads, by the braking system, at all times, so there is a constant and noticeable "drag" on the car.

The independent specialist I use has never seen this before and is at a loss to explain it.

He has checked that calipers and pins etc are free to move, plus taken the parking brake apart to check that wasn't defective.

It's all a bit puzzling......

 

[sinewave]

Has your Indie checked for failed dust caps and hence corrosion on the pistons?

If the Dust caps have let some shyte/water in then the pistons may well have slight corrosion on them which will not be visible without total removal.
This corrosion will stop the pistons from fully releasing their pressure off the discs and hence cause the drag you're experiencing.


(Don't view the SBC pump as an expensive mistake mate, it'll have fcuked up anyway sometime soon so you'll have had to have it done anyway!)

 

[trapperjohn]

Has your Indie checked for failed dust caps and hence corrosion on the pistons?

If the Dust caps have let some shyte/water in then the pistons may well have slight corrosion on them which will not be visible without total removal.
This corrosion will stop the pistons from fully releasing their pressure off the discs and hence cause the drag you're experiencing.


(Don't view the SBC pump as an expensive mistake mate, it'll have fcuked up anyway sometime soon so you'll have had to have it done anyway!)

Quality stuff from sinewave there. I'm guessing the car is a keeper or you would have chopped it by now. You and your indie and the forum will bottom this for you.

 

[SL300-24]

I think that's what I said in post 7.........corrosion of the caliper pistons, causes similar problems on motorcycles. Strip them right down and rebuild them with new seals, clean or renew the pistons and make sure they are moving back and forward easily.

 

[Merc Owner 2B]

Do the wheels drag when the SBC is deactivated?

 

[BlackC55]

There is another pump on the underside of the car too.

Also, us it still tight with the pads removed?

 

[Mark68]

Thanks for your help everyone - it's greatly appreciated.

Apparently it's when the SBC kicks in that the pads are forced on to the disks, with it then being very difficult to turn the wheels by hand ( which is consistent with the "drag" experienced when driving the car ).

As I'm not working on the car myself - unfortunately I don't possess either the skills or the tools to do that ! - I will need to share your ideas with and the indie in question, who is a good guy.

Thanks again,
Mark68