W164 Sudden overheating brake caliper/wheel hub?

[Richard Moakes]

Took the ML out for an airing today, all seemed fine until on the way home when I couldn’t avoid one the gazillions of potholes in our roads, no immediate problem, but shortly afterwards the one of the front brakes starts to squeal. Got home to find that wheel extremely hot and assumed it must be the brakes sticking.

Removed the pads, pushed back the pistons with no problem, everything was a little crusty as you might expect but cleaned everything up, lubricated all the sliding elements with plastilube, the sliding pins were dirty but definitely not seized. Put everything back together and the caliper was sliding freely until I took up the slack by pressing the brake pedal.

Took it out again, brakes pulled up evenly, thought I must have got it, but that wheel is still getting much hotter >100degC vs. 40degC than the opposite side, so I think there must be another issue.

The pads were nice and evenly worn, no excessive dust or glazing, so it started me wondering if the pothole had damaged the wheel bearing, it’s not excessively noisy but then again W164 is always a bit noisy so hard to tell. Can’t spin the wheel freely with it being 4x4 so not easy to spin the hub and listen.

Only other thing I can think is that a month ago it had the brake booster replaced under recall, could something have gone wrong there? I don’t see how it could?

Any thoughts or things to check would be much appreciated before I fire the parts cannon.

 

[Snake Charmer]

First thought is corrosion on the pistons or under the seal or collapsing flexible. What did you push the pistons back with?
If the servo has adjustment on the cylinder pushrod and has not been set correctly, there could be residual pressure, that normally affects both or all calipers.

 

[Richard Moakes]

I used a caliper spreader tool like this, did wonder about the flexi brake hose, cheap enough to replace

 

[Richard Moakes]

I used a caliper spreader tool like this, did wonder about the flexi brake hose, cheap enough to replace

View attachment 82816

Piston dust seals looked in very good condition.

 

[Snake Charmer]

I have a spreader like that, moves stuck pistons quite easily if you can get the dam thing in the caliper as it is quite wide.
What is the friction comparison on the opposite wheel after pumping the pedal?

 

[Richard Moakes]

Decided to change the flexi line and caliper, way cheaper and easier than stripping everything down to replace the bearing.

Frustrated that as it’s a 4x4, and I can’t get all 4 wheels in the air, I can’t easily check for brake drag by spinning the wheel without removing the driveshaft.

 

[mercmancdi]

Might be bit of pain but you could swap pads from one wheel to other then piston would be starting off in different positions in case it’s sticky at a particular point , and while pads are off get the cylinders well freed again , in out in out many time without popping one right out put bits of wood in. I would keep working at checking brake before going to wheel bearing.

 

[Blobcat]

Might be bit of pain but you could swap pads from one wheel to other then piston would be starting off in different positions in case it’s sticky at a particular point , and while pads are off get the cylinders well freed again , in out in out many time without popping one right out put bits of wood in. I would keep working at checking brake before going to wheel bearing.

The pads will have worn to the discs so swopping them left to right will give poor braking until they bed in - unless they're very worn then they'd be very unlikely to bed in correctly

 

[mercmancdi]

The pads will have worn to the discs so swopping them left to right will give poor braking until they bed in - unless they're very worn then they'd be very unlikely to bed in correctly

Agree completely but would , still work and let you see if it stops overheating

 

[DREAMER NO2]

Richard it could be the other brake caliper that is not working as it should ..So the one you have worked on will have to do more work to stop the car Did you look at the other caliper thats cool ?

 

[ajlsl600]

Get a proper flex pipe clamp tho I hav used mole grips and cardboard. Take car to straight quiet road clamp the pipe, with a good grip on steering whell and modest speed, apply the brakes. Do you hear or not hear the previous noise? Do you note heat?
Wud have thought, if booster out of ad or faulty it would effect more than one corner (wheel heating)

 

[LostKiwi]

Unlikely to be the booster as that affects the entire braking system.

 

[Richard Moakes]

Thanks for all your ideas, it’s much appreciated

There was no brake imbalance when slowing down, the disc was turning blue on the affected side and the wheel was very hot, whereas the opposite side was normal, i.e. Warm disc but not too hot etc.

Right now the plan is replace flexi line and calliper, parts have already arrived, got new pads and discs already in stock if I decide to go “all-in“ and replace everything

 

[ajlsl600]

If disc going blue u sure have issue, remove caliper try to blow pistons out with airline, split caliper use 24o grit clean bores, after use same paper n polish pistons. Unless you see real damage.
It is possible, but unlikely that flex has damage and is not letting fluid back.
If rota going blue. Check run out with magnetic dti as likely warped.

 

[mioba]

Take caliper off and get it tested - plenty of calliper places about. Best get other side done too if it needs a refurb.
If the sliders are free then then the pistons are sticking.
Very odd mind. Never had such an issue on a merc.
Callipers usually sieze due to non use

 

[ajlsl600]

You can get the kits. V easy job.

 

[EmilysDad]

Take caliper off and get it tested - plenty of calliper places about. Best get other side done too if it needs a refurb.
If the sliders are free then then the pistons are sticking.
Very odd mind. Never had such an issue on a merc.
Callipers usually sieze due to non use

I had 2 sticky calipers on my R Class ....

 

[DREAMER NO2]

Dont use any grit on the piston bores. Polish the bore if you have to with fine paint polishing liquid But if it is rusty or marked, they will need replacing .Same goes for pistons . Dont use emery paper, or wet and dry paper on the parts . Also if disc was blue that will need replacing . Because it might be warped ..And another thing to take in to account . All this might be down to bad wheel bearings overheating.

 

[Richard Moakes]

Dont use any grit on the piston bores. Polish the bore if you have to with fine paint polishing liquid But if it is rusty or marked, they will need replacing .Same goes for pistons . Dont use emery paper, or wet and dry paper on the parts . Also if disc was blue that will need replacing . Because it might be warped ..And another thing to take in to account . All this might be down to bad wheel bearings overheating.

That’s my concern, just got a nagging worry a pothole took out the bearing

 

[ajlsl600]

I have never had issue usin 240 in caliper and after the same on piston, with a little clean diesel. In 40 yrs of doing so. Just don't go at it more than req to clean em up. Then wash all in a little clean petrol and blow clean, dry . Seal kits normally come with small brake assly grease.
I have never had to repeat this, on any I have done, of course if pistons, chrome is off. It's a new piston or caliper issue. I have never had to replace a caliper yet.