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6.9 brake problems - getting desperate here....

Started by s class, 13 September 2010, 03:09 PM

WGB

Just adding an extra ha'penny worth.

After my recent rebuild of my 6.9 brakes I was not impressed by either the long travel feel of the pedal or the turbidity of the response.

My 450 on the other hand has a firm pedal with an almost abrupt response and you have to be careful not too push to hard or it will easily lock the front wheels.

The two cars have exactly identical braking systems down to the same pads fitted.

Now that I have been driving my 6.9 a bit more and actually covered 300km in the past month the pedal in it has firmed up nicely and there is a good feel of bite and progression and I am very happy with them - but it did take 300km or more of driving in the hills for them to bed in.

Bill

koan

Quote from: WGB on 06 October 2010, 11:34 PM
Now that I have been driving my 6.9 a bit more and actually covered 300km in the past month the pedal in it has firmed up nicely and there is a good feel of bite and progression and I am very happy with them - but it did take 300km or more of driving in the hills for them to bed in.

The same thing occurred to me yesterday, I was out driving and realised the brakes had improved quite a bit.

I've probably covered about 2000km, now pedal travel is no where near what it used to be immediately after the rebuild but it's still not as firm as I expect.

My farthings worth,

koan
Boogity, Boogity, Boogity, Amen!

s class

THE PROBLEM IS SOLVED

Guys, it turns out that the front caliper pistons were pulling back too far on pedal release.  The cause was apparently due to the new caliper seals gripping the pistons too well.  All piston motion was due to 'rolling' or 'stretching' of the seals.  There was no piston-moving-through-the-seals motion.  I believe this is what several of us have battled with.

What the guys at the brake place did was to remove the brake pads one at a time, and have an assistant pump the pedal to extend the piston to the 'worn-pad' position.  Then the piston was manually pushed back into the caliper.  The process was repeated 3-4 times on each piston to help the seals to wear in a bit.  Once reassembled, a vigorous test drive was carried out.

Now the braking is firm, immediated and positive. 


[color=blue]'76 6.9 Euro[/color], [color=red]'78 6.9 AMG[/color], '80 280SE, [color=brown]'74 350SE[/color], [color=black]'82 500SEL euro full hydro, '83 500SEL euro full hydro [/color], '81 500SL

wbrian63

How on earth did you ever figure this one out? I wouldn't have thought such a thing was possible. Maybe the "rebuilt" calipers used a "close enough" seal?

My deep experience with disk brakes is on US domestic autos, most of which are single piston, floating caliper in design. On those units, there's an internal o-ring like seal, which is square in section, that serves as the pressure seal. Then there's an outer boot that is attached to the caliper body, and tightly wraps the piston. This is the dust boot, and its design provides extra flexibility through accordion pleats to allow it to extend as the brake pads wear.

Does MB not use a similar design?
W. Brian Fogarty

'12 S550 (W221)
'76 450SEL 6.9 Euro #521
'02 S55 AMG (W220) - sold
'76 450SEL 6.9 Euro #1164 - parted out

"Bond reflected that good Americans were fine people, and most of them seemed to come from Texas..." Casino Royale, Chapter V

s class

The MB's of this era do not use floating calipers.  They are fixed calipers with a piston in each half.  Each caliper has a pressure seal (square cross-section) and a dust seal.  The square cross-section seal is the one that does the twisting or stretching as the piston moves.

The calipers were rebuilt twice in our efforts to trace the problems.  The caliper halves were meticulously prepared.  The seals are of local manufacture - I've used the same kits in several of my cars with great success.  I feel the quality is at least as good as what MB supplies.  I really don't think there is any issue with the seal quality or fit accuracy.  The brake specialists claim they see this problem from time to time - not exactly common, but they did almost immediately diagnose it. 


[color=blue]'76 6.9 Euro[/color], [color=red]'78 6.9 AMG[/color], '80 280SE, [color=brown]'74 350SE[/color], [color=black]'82 500SEL euro full hydro, '83 500SEL euro full hydro [/color], '81 500SL

koan

This good news, a lot of 116s are going to get this fix.

We had a long thread a couple of years ago that talked about this, I'd observed what I called "pad pull-back" while doing some bleeding on my car, something I'd not seen on other cars, no idea on how to fix it - or if that was the real cause.

I came across a specification for brake pistons, finish was to be dull chrome. I polished up my pistons. My thinking now is that the high polish doesn't allow fluid to be retained on the surfaces between the seal and the piston, this lack of lubrication makes the piston stick rather than slide.

Have to replace a front wheel bearing soon, while I'm at it I'll "un-polish" the piston with 800 or 1200 emery paper. Fortunately the bad bearing is on the side where I saw the worst pad pull-back. Hoping for significantly harder pedal.

koan
Boogity, Boogity, Boogity, Amen!

WGB

Great thread.

thanks for the follow up info S class.

Bill

craigb

Fantastic! This sounds totally logical to me and such an easy fix. I have just done calipers on a W108 and noticed that some pistons were easier to push back in than others. Before I bolt them on I might pop them back out and in a few times before fitting. With all that discussion around Oscar's car, I reckon Koan was really close to getting there, but as he says wasn't sure or how to fix it. Even if you had to have someone on the pedal and repeat 20 times pressing with an old worn pad in there and then returning it, eventually you are going to reach the stage where that piston and seal is not grabbing and maybe slight edges on the square seal softened and that slight amount of fluid to 'lubricate'. Clearly worked on yours and can see Oscar under the guards this very weekend! Actually I haven't noticed a reply......not sobbing on your keyboard after your Herculean efforts Simon.........it was a learning experience for us all!
1980 280s

TJ 450

In keeping with the "use theory", I've piled on about 5000km in the 6.9, and the brakes are perfect. I am due for a fluid change though.

I'll keep that in mind though, regarding roughing up the finish ever so slightly... I'm glad all is good now.

Tim
1976 450SEL 6.9 1432
1969 300SEL 6.3 1394
2003 ML500

s class

I don't know if I'm doing it right or wrong, but I have lately been preparing caliper pistons by cleaning them with a green kitchen pot scourer.  These are like a synthetic mat about 5mm thick, not the synthetic replacement for wire wool type.  It does a good job of cleaning off the rust marks and leaves an even slightly dulled finish. 


[color=blue]'76 6.9 Euro[/color], [color=red]'78 6.9 AMG[/color], '80 280SE, [color=brown]'74 350SE[/color], [color=black]'82 500SEL euro full hydro, '83 500SEL euro full hydro [/color], '81 500SL

oscar

Sorry guys for catching up late.  Firstly, congrats sclass.  It's completely logical and it's awsome to hear the brakes are still holding up well.

Craig, I still haven't touched my brakes for what must be close to a year now.  Can't afford the time at the moment.  I do remember koan mentioning pad pull back at one stage and I do remember viewing it on the fronts and tried pumping the pistons out and resetting the pads so they were more than just dragging on the discs but I doubt my method was as thorough as sclass' workshop guys.  For example, I didn't do repeated in and out movements to wear the seal in.  I'll give it another go at some point in the new year.  I'm starting from scractch with povo to give it w126 brakes but will try this fix again before the upgrade happens.

Just one question for you all about polishing/buffing/sanding the pistons.  If we are to scuff the piston sides producing a dull finish should the direction be circular, cross hatch, in the direction of piston movement, circumfrential or does it really matter?  I'm assuming the scuffing pattern produces lubrication channels similar to honed engine cylinders and master cylinders and just wondered what people's thoughts are before I attack pistons. 


1973 350SE, my first & fave

koan

Quote from: oscar on 30 November 2010, 09:29 PM
Just one question for you all about polishing/buffing/sanding the pistons.  If we are to scuff the piston sides producing a dull finish should the direction be circular, cross hatch, in the direction of piston movement, circumfrential or does it really matter?

I've always polished or emoried (?) surfaces in the direction of motion, around a rotating shaft where a seal runs but along a sliding surface.

I would think along a brake piston, not around it but not sure.

Does it matter?

I don't know.

koan
Boogity, Boogity, Boogity, Amen!

craigb

I would have thought that if you use a fine enough abrasive it shouldn't matter too much, but personally I reckon I would find the most worn pads I could find and just mess about pushing the pistons in and then pushing them out. Also sounds like clocking up plenty of km's will do it eventually, but wondering if the sitting will make it a bit worse, but still it has to give eventually. I just hate the fact you put so much into it and we couldn't help earlier, but still best to work it out now. And you know it is the gen 2 w126 that are the best ones? but doesn't matter if it is 300 or 560, they will be the same. and also the piston diameter is the same so your master is fine, just slightly bigger diameter disc and heaps more width/ventilation.
1980 280s