do I need to replace this caliper?

[jan lauwers]

Gentlemen;

Now that I have decided to replace the MC and the flex brake tubes, I am taking a critical look at the rest of the braking system. I have just removed a caliper, (Rear Left) and I have a feeling that the piston needs to be replaced. The surface which pushes against the back of the brake pad is not flat. Attached pics show where it is not flat (where the screw driver points), plus, the rim looks damaged, plus, it is all rather greasy. Bad news I guess?
I have tried to remove the 2 pistons by applying 4-5 bar via the inlet of the fluid. They don't move. Is this normal? Haynes says that not much pressure is required to make them come out... so does that mean that they did not move under the pressure of the brake fluid either? The other pic shows the back of the brake pad, and there too, the uneven surface of the piston(s) is evident.

my questions:
What has caused this uneven surface?
Why are these pistons stuck and does it mean that I need to replace the whole caliper(s)??


Thanks again gents!

[jameshoward]

You could try spraying some Plus Gas (or something similar) around the caliper to free it up. It's outstanding and will free it unless it's mechanically jammed in there. The caliper should move relatively freely. 4 bar should most certainly move it. I'd give Plus Gas a try before jamming a screwdriver in there.

If you do elect to replace the caliper, you will be able to return the existing one, which will save you a lot of money. You could also replace the piston seals on the caliper and rebuild it if you were so minded. It would be far cheaper if you know how.

[Raymond]

Starting is important, stopping is essential.  I'd replace both calipers.  If this one is toast, the other probably isn't far behind.  In an emergency, having both brake sets equal can make the difference between avoidance and impact.  Brakes are the one system where cutting corners to save a couple of dollars is a genuinely poor investment.

[Cees Klumper]

My old calipers have the same uneven / indented surface, so that seems normal. I replaced my four calipers €1,000, ouch) two years ago. Braking is much better now. Agree on the safety point. You could shop around to see what handing in your old calipers will save you. I didn't so still have my old ones sitting around.

[GGR]

The pistons should definitely pop out with air pressure. If not your brakes do not work as they should. Exchanging them for rebuilt ones is quite affordable, making the work to rebuild them not too justified (But hey, if you rebuild them yourself, at least you're sure of what you have).

Your front calipers are the most important ones, they do most of the braking. So have a look at them too if not done already.

Finally, it may be a good idea to replace the rubber hoses too: They disintegrate from inside and end up acting as a one way valve. pressure goes through when you step on the pedal but then fluid is not making its way back with the result of residual braking being applied when it shouldn't. 

Oh, and there is also a proportioning valve on the Pagodas that can act weird sometimes. It is in the back, between the two rear brake lines.

[jan lauwers]

Thanks everyone.

As always, all very useful information. I think I will look around if I can find new or rebuilt ones and trade mine in. Sounds like the best balance between safety and cost. And yes I will do the front ones too.

Cees says his are uneven too. If so, does any one know why that is? I don't see the point if the backs of the brake pads are even...??

Jan

[WRe]

Hi,
I have overhauled my rear calipers last year. It was not difficult but very time consuming, because you have to work very accurate and careful, e.g. to get the pistons out with air pressure. They will come out like a canon ball, watch your fingers! Also to mount the dust gaiter is pretty bothersome. I would do it nevermore. Compared to the price of a new caliper (~ 150€) it's not worth, because the old ones stay old and the Mercedes calipers has always been a mess.
...WRe

[450sl]

Actually it can be quite dangerous using air.

Better go and make yourself an fixing, to connect your greasegun to the caliper , it delivers much more and safe pressure.

Using this method i have removed dead stuck pistons without any damage, like a dream.

[GGR]

That looks to be a nice trick!

Would the grease gun fit on the bleeder? If not, would a greasing nipple as found on the front axle fit in place of the bleeder? If not, how did you go about it?

[stickandrudderman]

Let us assume that at a given speed your stopping distance with a perfect braking system is 100m.
Now, let us assume that one piston is seized in a rear caliper. This will give about 5% reduction in braking efficiency which in turn will increase your braking distance by 5m or more than the length of a large car.
So you've gone from not having an accident at all (just a laundry bill) to totally ruining your day and someone else's.
Please, don't mess around when it comes to yours and other's safety and keep your braking system in top condition.

Here endeth today's sermon!

[450sl]

Luckely i am in the position to make my own specials , for instance for the frozen piston i manufactured a blank to replace the cil.head
and made a fitting to accomodate the greasegun. First i filled up the cilinder with cocacola and the topped it off using the greasegun.

I have also used a brakepump from a motorbike to freeup brakecalipers , but the brakefluid is not very userfriendly.

When there are no spares available , i have made new pistons out of highgrade stainless to suit the calipers



 

[jan lauwers]

I am going to try and trade mine in. I have now heard that this is possible through the normal Mercedes dealership network. I am very curious what the cost will be compared to buying from SGS or Niemoller.

The question however about the form of these pistons keeps fascinating me. Why are they not plat if the back of the pads is flat? What's the benefit of that 'step' in their surface? In the mean time I have found out that it has to do with a rotation under a certain angle in the cylinder... but then... what's the benefit of them sitting there rotated under a certain angle?

Who knows this?

Thanks guys!

Jan

[jaymanek]

My rears started over heating last year, replaced both with ATE replacements.

Then this year my front right over heated on a short journey.
Replaced both fronts along with the short copper pipes (front axle) and all flexible pipes.

These calipers arent big money, I would replace them all for safety.

You can also have yours rebuilt, again very cheap and not worth messing around with in my opinion.

MB Calipers are quite expensive, you can get same items after market.

[GGR]

overheating brakes are not necessarily due to binding calipers. It can be due to old hoses as per described earlier. Changing both surely solves the problem, but it is preferable to diagnose precisely what the issue is so as not to replace good parts. The way to tell is to see if the piston can be pushed back easily when opening the bleed. If so the culprit is the hose.

[stickandrudderman]

My rears started over heating last year, replaced both with ATE replacements.

I once drove a 107 the two miles to the MOT station and when I arrived (for an MOT) the rear brakes were on fire!

The master cylinder was the culprit.

[jaymanek]

In my case the cause was certainly sticking pistons... They were solid on one side.

I know when the last owner bought the car he had to have all the brakes "serviced" to get them working. I guess this meant just freeing them off instead of replacing or refurbishing them.

My hoses were like new.. I changed them anyway just to be sure, as the over heating may have damaged them inside.