Left rear brake not releasing properly update - problem solved!

[Cees Klumper]

My car had not been driven regularly over the past two years. I recently started driving it again, and noticed that my left rear brake is 'hanging up' meaning one or both of the brake pads won't release the disc properly, which creates a noticeable drag as well as some heat build-up on the disc and wheel (although not extreme). Sometimes it will release, but most of the time it sticks. I have replaced the brake hoses just a few years ago and before putting the car away two years ago this problem was not there.
I just took out the brake pads, cleaned them and the caliper so that the pads move freely in the caliper, but after applying the brake and releasing it, the pads stay pressured against the disc as before.
I've read up on the brake proportioning valve but that would not appear to be the cause. So, since the hoses are new and the pads move freely in the caliper, I have by process of elimination concluded it must be the caliper itself - perhaps some corrosion in the pistons. The storage was a bit humid. Before I order new calipers, am I overlooking something? Could it be the proportioning valve after all? The other three brakes work perfectly. It's also not the parking brake. Thanks for any comments and suggestions.

[twistedtree]

Are you able to retract the caliper pistons?  I would think that would give a good indication whether things are binding up.

Also, when the brake is binding, you might try cracking the brake line and see if the caliper releases.  That would tell you if it's the caliper or back-pressure in the hydraulics.

[Cees Klumper]

Thanks Peter. I'm a bit stumped on this one:

- I am able to push back the pads so the pistons are not stuck. However, on applying the brake, the pads bind again and do not move free out of themselves
- I did loosen the bleed nipple at one point and no fluid came out

All symptoms point to the brake hose, however I did replace those around 2007, when I also had the brake fluid replaced. I can't imagine in those few years the hose became restricted again, although I could double check I suppose.

I saw your participation in many posts concerning the proportioning valve - do you think a malfunction there could explain these symptoms? It is clearly letting fluid through and, as far as I understand, it would not normally malfunction in a way that would make it act as a one-way valve.

[jacovdw]

Hello Cees,

Regarding the brake flex hoses - new hoses from MB actually come with an expiry date printed on them (5 year shelf life).
Even the BBB recommends replacement every 5 years...

However, another possibility to look at would be the caliper itself.
There seems to be a tendency for rust to eventually form just under the dust seal for the caliper pistons (especially if the car is not driven regularly and the brake fluid not changed annually).
The area that is affected is the bore between the square piston seal and the dust cover.

Any build-up of crud will result in a piston being sluggish to return to it's rest position when the brake pedal is released.

I took some pictures last year when I did the brake system on my 114 and as the ATE calipers are basically the same, I will post some of the photos here later.

EDIT: photos attached.

The first photo shows the expiry date on the flex hose packaging label and the second photo of the offending rear caliper.

[twistedtree]

It is a puzzler.  It sounds like you can reproduce it on a lift or with the wheel jacked up?

I'll apologize in advance if I say things you already know.....

With disk brakes there is no return spring so the pads don't retract, they just stop exerting force against the rotor.  As a result, some initial drag is normal after the brake is released.

Because pad movement is miniscule when the brake is released, I wouldn't expect any quantity of fluid to be released by cracking the brake line fitting.  However, if there is back pressure, the brake would then release.  Opening the bleed screw is probably the easiest way to conduct the test - then you can be sure you are releasing pressure right at the caliper.

If this is what you've already done, then I'm scratching my head too.  It's hard to understand why the caliper would be binding up, yet you are still able to manually retract the pads/piston.  And just to confirm, are you able to move each piston in without the other moving out?  If they only move in equal and opposite directions it might be another indication of a fluid restriction.  You'll have to hold the opposing piston somehow, but then you should be able to retract each on it's own.

The temptation is certainly there to replace/build the caliper, but personally I hate doing expensive things like that without clear evidence it's the problem.  Almost always when I have conflicting evidence about the source of a problem, it turns out that it's not the problem.  It always makes sense when you look back at it - it's that darn foresight that's the problem.

As for the proportioning valve, I have to say I'm drawing a blank on that one.  Perhaps a different person made the contributions you are thinking about?  I know very little about them, so it's unlikely I provided any real insight.

[Peter van Es]

Cees,

do you remember I had a fuel hose that went bad after just 2 yrs? Had cracks in it and leaked like crazy. Apparently a bad batch, cause van Dijk had encountered this before. Perhaps you shouldn't trust that brake hose just yet!

Peter

[UKSteve]

Hi,
I have had exactly this issue, although it was with a 1967 230 which has rear drums. it was quite puzzling at the time.
It turned out to be the hose, which is an easy fix. Hoses can can behave in strange ways without presenting an externaly visible clue.
They can close up enough to behave like a valve and maintain pressure in the wheel cylinder, although often you are able to manually move the cylinders back "relatively" easily.
I have seen similar posts elsewhere on the forum.
Best regards.
Steve

[stickandrudderman]

A sticky caliper can, in most cases, be resolved thus:
Remove one brake pad and replace it with a shim of similar thickness to a thoroughly worn out pad (around 6mm), a peice of wood will do. It doesn't have to be anywhere near the same size as the pad, it's only to prevent the piston from leaving it's bore in the caliper in the next step.
Now, push the brake pedal to force the piston out of the caliper and against the shim.
Use whatever tool you normally use to then push the piston back into the caliper as far as it will go.
Repeat a few times.
This often cleans up the surface of the piston and liberates it from its sticky bonds.
You'll know if it's working because the force required to move it back into the caliper will be much less than when you started.
Re-install the brake pad and repeat for the other pistons as necessary.
It doesn't work every time but over the years it's worked for me a lot.

[Cees Klumper]

Thanks so much for all the very helpful response! Here's the plan:

First, Stickandrudderman tip, because I think it's the most likely suspect
Then, if that does not solve it, replacing the hoses
As a last resort, the proportioning valve and 'caliper-proper'

Stay tuned ...

[Atazman]

How about just bleeding pressure by opening the bleeder?  If the brake frees up, the cause is likely the hose holding pressure.  If the brake does not free up, the cause is likely the stuck caliper as already suggested.

Just my suggestion.........

[Cees Klumper]

Hi Atazman - yes, I did at one point crack the bleed nipple and no fluid came out, indicating to me at least that there was no pressure coming from the hose or further 'upstream' going into the caliper - that's why I was thinking it would be the caliper or pads themselves.

[twistedtree]

Cees, even with back pressure, I wouldn't expect much if any fluid flow when opening the bleed screw.  The whole brake system is designed so nothing expands under pressure, including the hoses.  Obviously there is some minimal expansion, but it's VERY little - otherwise the brakes would be spongy.  Air bubbles make the brakes spongy because the tiny bubble can be compressed, where the fluid cannot.  The only way you would get fluid flow is if something has been compressed and re-expands when you crack the fitting. Unless you have air in the system, there should be no appreciable expansion/contraction when you apply and release the brakes.

I think when you do the back-pressure test, rather than judging the outcome by the amount of fluid that escapes the bleed nipple, you need to judge the outcome by whether the wheel spins more freely than before.

[Benz Dr.]

There should be some pressure in the system at all times or the check valve in the master cylinder isn't working. It's not much pressure but you want all little so that the lines remain charged while not in use. If you open a bleed screw there should always be some fluid present.

There are return springs on the brake pads. They're called cross springs and you're brakes worn't work all that well without them. We remove, clean, and lubricate all of the parts that go into the calipers such as the pads, pins and cross springs. The pads have to be loose enough to slide into the caliper by hand or they're too tight.

The brake pressure valve at the back of the car should have zero effect while bleeding the brakes. Inside of the valve, there's a sleve with a pin inside of it. As the brake pressure builds, it starts to bush on the pin, which shuts off any brake pressure to the rear calipers. The harder you push on the pedal, the more force is applied to the pin. The front calipers will eventually lock up but the rears should allways pull down and never lock up. Since the car's weight will be transferred to the front while under heavy braking, less and less pressure is required at the rear with increasing brake pressures. Without this valve, the rears would always lock up first sending you right into the pumpkin patch. Clever idea that really works.

Rear drum brakes won't have this valve. Braking pressure is regulated by the size of the brake cylinders and the fact that rear drums won't have as much braking effect as disc brakes. Try backing up at 20 MPH and see how well rear brake drums stop the car. With only one leading shoe and a lot of the cars weight bearing down on it, you better have a clear path ahead.

[Atazman]

Cees......

Yes, I knew that you had already opened the bleeder, and that the caliper/pistons/pads  were still hung up.  I mentioned it again only to affirm what you already were suspecting.... the problem lies within the brakes themselves (either the piston sticking or the pads hanging up). 

Mystery solved.

Please keep us all informed of what you end up doing.

[Cees Klumper]

And here is the outcome ... Stick is the man ...

I did exactly as he suggested: pulled out one of the brake pads, pushed the piston back in all the way (which was not that far considering the pads are almost new, so the pistons are at their 'most-inward' point), put in a 'filler' the tickness of a maximum-worn brake pad, and applied the brake until the filler contacted the disc (and it did stay pressed firmly against the disc, also after having released the brake pedal). Then I pushed the piston all the way back in - that was very difficult. I repeated that procedure about 4 times and, as Stick predicted might happen, each time became easier until the filler would come loose after relasing the pedal - just like the brake pad should, as the Dr mentioned. I put the pad back in and confirmed that the brake is functioning normally again - problem solved!

Thanks to all for all the tips ...

[wwheeler]

Great news! That is what happened to one my front calipers. I did the same back and forth proccedure and luckily it loosened up. I believe it took me about 10 or 15 times before it was normal. The first few times as you say were a bear to get the piston to move back.

For an old car like ours that may not be driven as much, I think it is a good idea to raise each wheel and check the ease of rotation once a year to be sure the brakes haven't locked. I just happened to discover my piston seizure when I had the car on a lift for a MBCA tech session. Lucky I found it.

[twistedtree]

great news, and a nice technique to remember....

[Naj ✝︎]

Maybe you should consider changing the brake fluid as well...

Naj

[Cees Klumper]

Hi Naj - for sure, that is high on the list, along with coolant hoses and fan belts.

[stickandrudderman]

next time I'm here:
http://www.restaurant-entrecote.ch/home.html
I'll get them to send you the bill!