Long brake pedal.

Bellow

MB Enthusiast
Joined
Apr 26, 2010
Messages
6,692
Location
Ecosse.
Car
C2500 350, 450
Just sniffing out some ideas here on what is currently an obscure problem.
Working on the front wheel bearings required caliper removal. While I was there I squeezed the piston in and back out again with the pedal a couple of times each side (taking steps to ensure no piston popped out). Left the pistons 'in' for caliper re-fitting but when I went to pump the pedal to seat the pads against the disc the pedal didn't firm up as expected. Eye-balled the pads and calipers and the pads were snugged up to the disc so assumed it must need servo (actually driven from PAS pump) to give me the pedal.
Not so, as on starting it up there is still a long and mushy pedal. Feels like they need bled (but the hydraulics were never broken into and the reservoir level never dropped) which I will do anyway. The other possibility is a damaged master cylinder piston seal but, the brakes work very well (after a test drive that I gave the brakes a decent work-out on) and, when I stand on the pedal although it goes much further that I'm happy with, it still does come to a hydraulic stop - not the floor. That I doubt is possible with a failed master cyl.
Needing bled is the most obvious thought but without breaking the hydraulics, how could air have entered? There are no external leaks or any other untoward signs. Any ideas?
 

190

MB Enthusiast
Joined
Mar 4, 2015
Messages
3,803
Location
Cheshire
Car
2009 W204 C180K
Did you remove the pads and if so did they go back on the side of the caliper they came out. Sounds bizarre but I've had pads wear slightly tapered and putting them back on the wrong side caused a small but noticeable increase in travel. This is a small effect so may not be your problem.

Air in the system aside, brake pedal travel is a function of how far the pistons are pulled back off the discs by the hysteresis in the piston seals. A very firm pedal can mean the piston isn't pulled back hardly at all because they are starting to seize in the caliper. By exercising the pistons as you did they may now pull back a little further which increases brake pedal travel.

Brake Seal.jpg
 

m80

MB Enthusiast
Joined
Apr 26, 2015
Messages
2,781
Location
Derbyshire, High Peak
Car
Vito 115 Lwb Dualiner.
I've had the same and managed recovery by hard brake action on a drive,
and bleeding when that wasn't enough recovery.

If honest I've not considered the theory behind it too much, but did wonder if had disturbed master cylinder seals.
 
OP
OP
Bellow

Bellow

MB Enthusiast
Joined
Apr 26, 2010
Messages
6,692
Location
Ecosse.
Car
C2500 350, 450
Did you remove the pads and if so did they go back on the side of the caliper they came out. Sounds bizarre but I've had pads wear slightly tapered and putting them back on the wrong side caused a small but noticeable increase in travel. This is a small effect so may not be your problem.

Air in the system aside, brake pedal travel is a function of how far the pistons are pulled back off the discs by the hysteresis in the piston seals. A very firm pedal can mean the piston isn't pulled back hardly at all because they are starting to seize in the caliper. By exercising the pistons as you did they may now pull back a little further which increases brake pedal travel.

View attachment 93243

Good point - thanks.
The pistons were kind of stiff - which reinforces the value of freeing them and I did give them a big push back. Pads went back where they came from (unusually, the inner and outer pads are very different. One is fat and round mimicking the piston, the outer long and slim like the slider. Both are clipped in).
As it is, they are super sharp but with the long pedal. And its tardy return.

There is another possibility - see next post.
 
OP
OP
Bellow

Bellow

MB Enthusiast
Joined
Apr 26, 2010
Messages
6,692
Location
Ecosse.
Car
C2500 350, 450
I've had the same and managed recovery by hard brake action on a drive,
and bleeding when that wasn't enough recovery.

I gave them a work out last night - sharp stops, and feet on two pedals to the point it's dragging down a 5.7l V8!

If honest I've not considered the theory behind it too much, but did wonder if had disturbed master cylinder seals.

Someone I spoke to today who knows this stuff reckons the pedal would be on the floor if the master was damaged in any way. It locks hydraulically before it reaches a physical stop (ie floor) and the brakes are effective - this thing weighs 2500kg.

Beginning to wonder if the flexibles didn't get damaged while I was working on the calipers. I was no rougher than on anything else I've ever worked on and never damaged a hose but someone before me could of gripped it previously - the killer for hoses IMO. Something else to look at. Maybe before bleeding - how can air get into an unopened system? Not plausible is it?
 
  • Like
Reactions: m80

Jobsworth

Active Member
Joined
Sep 23, 2018
Messages
413
Location
Horsham
Car
One or two...
I’m concerned about how far you “stroked” the master cylinder when you were pumping the pedal. I’m always wary of pushing the piston further down the bore than it would travel in normal use. There can be debris or corrosion down there that will remain undisturbed in normal use. Pushing the piston into that can damage the delicate rubber seal resulting in a long pedal. Its also possible that exercising the calipers has introduced air at that end. Bleeding will resolve quickly and simply if that us the case. Classically, air in the system will give a long pedal but it will pump up with two or three fast applications to a good, solid pedal. Master cylinder failure will pump up to a good pedal that gradually sinks as you hold the pressure on.
 

pmcgsmurf

MB Enthusiast
SUPPORTER
Joined
Feb 22, 2010
Messages
4,799
Location
Stirling, Scotland
Car
E55 AMG W211, E250 Sport W212, E220 Avant. W211 (and some Imps)
I'm curious why you felt the need to push the piston back out with the pedal?
 
Joined
Dec 7, 2015
Messages
30
Location
Westgate,kent
Car
Ml270 2003
Last time this happened to me after changing piston (it failed mot), I was told it was the back pressure had affected the abs unit(dirt/ air /seals) and the car needed bleeding with a diagnostic that could operate the abs and bleed it while bleeding brakes,, I was also told by another source to drive the car fast down a dirty track /grass farm rd and slam on the brakes to repeatedly activate the abs , which I did several times , seemed to improve but at a standstill pedal still crept to floor(brakes worked fine when driving), I found different mot station that couldn't do brake test on 4x4 and used a break testing device that sat on drivers seat while they drove car and braked, car past mot and I lived with brake creep.
wether the diagnostic bleed of abs(activates abs to bleed it while doing bleed sequence) would have worked I never found out ,
 
OP
OP
Bellow

Bellow

MB Enthusiast
Joined
Apr 26, 2010
Messages
6,692
Location
Ecosse.
Car
C2500 350, 450
I’m concerned about how far you “stroked” the master cylinder when you were pumping the pedal. I’m always wary of pushing the piston further down the bore than it would travel in normal use. There can be debris or corrosion down there that will remain undisturbed in normal use. Pushing the piston into that can damage the delicate rubber seal resulting in a long pedal.

I tried not to use more travel than absolutely necessary but had to to get the pads near the disc.


Its also possible that exercising the calipers has introduced air at that end. Bleeding will resolve quickly and simply if that us the case. Classically, air in the system will give a long pedal but it will pump up with two or three fast applications to a good, solid pedal. Master cylinder failure will pump up to a good pedal that gradually sinks as you hold the pressure on.

Pistons while being 'exercised' were never out further than disc thickness - less. There is no sinking with pressure on. I get a solid pedal then - just late in the travel.
It seems to be improving day by day even without being driven on the road. Piston seals I think are at the root of this.
 
OP
OP
Bellow

Bellow

MB Enthusiast
Joined
Apr 26, 2010
Messages
6,692
Location
Ecosse.
Car
C2500 350, 450
I'm curious why you felt the need to push the piston back out with the pedal?

Me too, but it’s done now

The only method I know. Is there another?
I did it during 'exercising' the piston but having pushed them back in, even if I didn't 'exercise' them, it would still have to have happened on first brake application when re-assembled. Which happened too.
 
OP
OP
Bellow

Bellow

MB Enthusiast
Joined
Apr 26, 2010
Messages
6,692
Location
Ecosse.
Car
C2500 350, 450
Last time this happened to me after changing piston (it failed mot), I was told it was the back pressure had affected the abs unit(dirt/ air /seals) and the car needed bleeding with a diagnostic that could operate the abs and bleed it while bleeding brakes,, I was also told by another source to drive the car fast down a dirty track /grass farm rd and slam on the brakes to repeatedly activate the abs , which I did several times , seemed to improve but at a standstill pedal still crept to floor(brakes worked fine when driving), I found different mot station that couldn't do brake test on 4x4 and used a break testing device that sat on drivers seat while they drove car and braked, car past mot and I lived with brake creep.
wether the diagnostic bleed of abs(activates abs to bleed it while doing bleed sequence) would have worked I never found out ,

On my other car where the ABS unit is close to a front brake I open the bleed nipple if I'm pushing a piston in for that reason. With this one, the ABS unit is a couple of feet away (vertically) and no chance of contamination with the small amount of fluid displaced relative to the pipe volumes.
 

pmcgsmurf

MB Enthusiast
SUPPORTER
Joined
Feb 22, 2010
Messages
4,799
Location
Stirling, Scotland
Car
E55 AMG W211, E250 Sport W212, E220 Avant. W211 (and some Imps)
The only method I know. Is there another?
I did it during 'exercising' the piston but having pushed them back in, even if I didn't 'exercise' them, it would still have to have happened on first brake application when re-assembled. Which happened too.

I get it but still don't understand why you felt the need to "exercise" the piston as you say while off the car ("taking steps to ensure no piston popped out") ?
 
OP
OP
Bellow

Bellow

MB Enthusiast
Joined
Apr 26, 2010
Messages
6,692
Location
Ecosse.
Car
C2500 350, 450
I get it but still don't understand why you felt the need to "exercise" the piston as you say while off the car ("taking steps to ensure no piston popped out") ?

We must have crossed wires here.
I wouldn't re-assemble/re-fit any caliper without first checking the piston and sliders are free. They are so seizure prone they need checking. The only way to pop the pistons back out (short of caliper on bench and compressed air) is use the car's hydraulics via the pedal. I removed the calipers from their mounts, but not from the car (to save breaking into the hydraulics).
Been doing this for ever and never before had a problem.

Steps to prevent piston ejection were suitably sized pieces of wood between the pads. Larger than disc thickness so final travel of piston not enacted until all re-assembly completed - whereupon the problem surfaced.
 

m80

MB Enthusiast
Joined
Apr 26, 2015
Messages
2,781
Location
Derbyshire, High Peak
Car
Vito 115 Lwb Dualiner.
I get it but still don't understand why you felt the need to "exercise" the piston as you say while off the car ("taking steps to ensure no piston popped out") ?

I would.
Come winter I experience brakes getting tight. Often rust on the sliders, but the pistons can become reluctant too.

Before the cold sets in I like to 'service' the brakes. That includes a clean up, copper grease to disc edges and exercising the piston. Gotta be careful to keep copper grease away from any rubber, it swells it.
If the piston is pushed out some and the dust cover removed it gives chance to clean any crud from near the seal.
I generally use the pump and lever technique but maybe it's better to release the bleed nipple as the piston is levered back in. A bleed of the caliper would then make sense.
 

pmcgsmurf

MB Enthusiast
SUPPORTER
Joined
Feb 22, 2010
Messages
4,799
Location
Stirling, Scotland
Car
E55 AMG W211, E250 Sport W212, E220 Avant. W211 (and some Imps)
We must have crossed wires here.
I wouldn't re-assemble/re-fit any caliper without first checking the piston and sliders are free. They are so seizure prone they need checking. The only way to pop the pistons back out (short of caliper on bench and compressed air) is use the car's hydraulics via the pedal. I removed the calipers from their mounts, but not from the car (to save breaking into the hydraulics).
Been doing this for ever and never before had a problem.

Steps to prevent piston ejection were suitably sized pieces of wood between the pads. Larger than disc thickness so final travel of piston not enacted until all re-assembly completed - whereupon the problem surfaced.

Get it now, it's just not something we ever do with the pistons, let's face it if the system was ok before the strip down it should be ok after re-assembly.
Sliders yes, we check these clean them, lubricate and re-assemble.
 

pmcgsmurf

MB Enthusiast
SUPPORTER
Joined
Feb 22, 2010
Messages
4,799
Location
Stirling, Scotland
Car
E55 AMG W211, E250 Sport W212, E220 Avant. W211 (and some Imps)
I would.
Come winter I experience brakes getting tight. Often rust on the sliders, but the pistons can become reluctant too.

Before the cold sets in I like to 'service' the brakes. That includes a clean up, copper grease to disc edges and exercising the piston. Gotta be careful to keep copper grease away from any rubber, it swells it.
If the piston is pushed out some and the dust cover removed it gives chance to clean any crud from near the seal.
I generally use the pump and lever technique but maybe it's better to release the bleed nipple as the piston is levered back in. A bleed of the caliper would then make sense.

See what you mean, we would do this along with a fluid change every other year, not yearly.
Stopped using copper grease a long time ago, we only use ceramic grease now due to it's no conductive properties (like copper grease).
 
OP
OP
Bellow

Bellow

MB Enthusiast
Joined
Apr 26, 2010
Messages
6,692
Location
Ecosse.
Car
C2500 350, 450
Get it now, it's just not something we ever do with the pistons,let's face it if the system was ok before the strip down it should be ok after re-assembly.
Sliders yes, we check these clean them, lubricate and re-assemble.

Was it OK though? This one is new to me so no idea what is and what isn't right.
On a more familiar car things creep up incrementally unnoticed. With my smart, braking into a downhill bend in the wet the tail offered to step out. Caught it before ESP did (I'd been karting in the wet the previous week - on slicks) but later investigation found a seized front caliper slider not doing its share.
I just assume brakes will seize - its what they do.
 

m80

MB Enthusiast
Joined
Apr 26, 2015
Messages
2,781
Location
Derbyshire, High Peak
Car
Vito 115 Lwb Dualiner.
See what you mean, we would do this along with a fluid change every other year, not yearly.

Ideally, but we have a Scottish type climate here. Loads of salt in winter, loads of wet and humid weather. Stuff deteriorates and rusts. So I've learned to give these things the service before the cold weather arrives. I've had too many MOT's in colder months and the brake imbalance is then something I've a problem with. Strangely Larisa's Gr Cherokee, that I drive seldom, can fail on imbalance and it isn't noticeable in normal use.

Oh, copper grease is better on the rusting pad edges I think, but I tend to apply graphite there from a joiners pencil first.
 
OP
OP
Bellow

Bellow

MB Enthusiast
Joined
Apr 26, 2010
Messages
6,692
Location
Ecosse.
Car
C2500 350, 450
Problem seems to have sorted itself out though I need to try it with its assistance (ie engine running) to be certain.
I think the seals were the problem (and will probably have a look to check) but I've also got a heads-up on the GMT 400 forum to check PAS fluid level. This can cause odd effects with the brakes apparently.
 

Users who are viewing this thread

Top Bottom