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Drivers side brake caliper pistons stuck out (only on one side of the caliper)

30K views 53 replies 18 participants last post by  TRUKTOR 
#1 ·
So I'm trying to get the brake caliper pistons on the drivers side to retract in preparation for the weekend rotor/pad replacement. The outer pistons seem to not want to budge... The inside ones I can push with my thumbs (with the bleeder open) but the outer ones are both stuck out!?!? what gives, I didn't want to crank to hard on them with a C-clamp until asking the masses

So its the outer piston on both the front and back caliper on the driver side, what gives? or rather, what doesn't give?
 

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#2 ·
Update:

I manged to get them to go in with a little more force than was first initially comfortable using (read, last set of calipers I retracted with a c-clamp were on a Chevrolet sprint, gotta think bigger on the mog)

With a slightly liberal application of a prybar and socket on the piston (to keep it going in straight) it went in, Fluid came shooting out with serious force (I had the bleeders completely removed) seemed almost plugged on the outer side!? :confused:

However, the drivers side rotor and outside brake pad were both worn down as well, on the outside where the pistons were stuck out, not near as bad as the passenger side, but I'm wondering if somehow the front brakes remained clamped on somehow, perhaps the 4 wheel park brake I use when winching didn't fully release in the front?

A full flex hose replacement is going down this weekend, so if its a collapsed flex hose, I'll get that with the swap, but I'm nervous the new rotors will fall victim to the same issue if its not a collapsed flex hose, and I can't afford doing this again anytime soon.

What all can stop a piston from retracting? Its buttery smooth now, I can move it with a finger again like the other ones....

Any idea's from the board would be, as always, greatly appreciated.
 
#3 ·
T,
I had a similar problem and pulled the calipers apart. Corrosion had eaten up the piston a good bit. A new piston and rebuild kit fixed it up and did not hurt as bad a I thought they would.

But Mine was not buttery smooth before the tear down. Maybe the piston was cocked in the pot?

C.
 
#6 ·
When I pushed on the free piston (inside) it looked like regular fluid

When I got the stuck (outside) ones free, it looked like Guinness beer, darker than it should have been, and there might have been some crud came out, a full flush is scheduled for the weekend as well.

Scott, do you guys still sell your power bleeders? I haven't had a chance to hunt down the parts to build one, I planned on gravity bleeding but perhaps a power flush is in order.

How does the dust boot come off so I can inspect it? My repair manual says "Remove dust boot" and shows it already removed, I'm thinking small flat blade screw driver and a steady hand?
 
#13 ·
Assuming ugly fluid I would pull it apart. (By that I mean piston/dust boot/seals) Then determine if the caliper halves need to be separated.

We stopped carrying the Motive Products Power Bleeders due to supply/customer service issues. Highly believe in the quality of the product, but for us as a reseller, the hassles of repeated customer service problems were not worth it. We refer people to order directly from them:

Around here we prefer the 0109 Black Label Euro Bleeder. If using it a LOT you can upgrade the reservoir to the larger version. Bleeders / Single Adapter Bleeders -- Motive Products Online Store

A small flat blade screw driver should pop the dust boot off. Most auto parts stores should have caliper assembly lube. I've got a couple of tubes around here but haven't ever bothered to sell it. Use it liberally under the dust boot as the farther your pistons extend the more the seal surface is exposed to the salt/debris. Looking at your photos of how far your pistons were out, I would expect that they will be toast if they stayed that way for long.

Cheers,

Scott
 
#7 · (Edited)
Good dust boots are a start, but I think that moisture/road-salt slurry can seep in and get to
work between the pistons and the bore. It does not take a lot to screw things up. It may not
be the case, but a look at your bore will tell you in a second.
Since it is only the outers, maybe the road-salt theory isn't it alone...I wonder if it might be a maintenance issue, and they were just not lubed up with the magic paste when last serviced. If you got them to move without backing anything off, then they must have been semi-stuck in the bore, and it was not an adjustment issue.
Makes me wonder if this is connected to the original problem...


Also- if you do wind up building your own power bleeder, most parts stores (down here anyway) carry the metric sized caps for the reservoirs.Just put the hose fitting on the spare cap, and you are good to go. There are not too many choices out there. If you walk in with your cap, you can probably match it. I walked into a small independent PORSCHE shop, and the guy had a pile of them. He was happy to be able to say that he "Now carried UNIMOG parts".
 

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#10 ·
It's kept under the counter at the Rathskeller....you have to ask for it....


Seriously, Though, I'd check Scott or Sean. It has appeared in photos of every single brake service thread I have ever seen posted on the German Sites, so they certainly believe in it. It is always the same tube...
 
#12 · (Edited)
Here is a thread regarding the paste- yet another BMW thread, but they ask the question.
Posts 7, 10, 11 seem the pertinent posts. It seems BMW specifies it, and hardly anybody uses it.
I only know about it because I saw it in so many UNIMOG brake photos and wondered what it was :

ATE brand Bremszylinderpaste.

Pegasus Racing, mentioned in the BMW thread, has been a good company to deal with in my experience.


ATE Brake Cylinder Paste - BMW MOA

ATE North America page :

http://www.ate-na.com/generator/www/us/en/ate/ate/general/home/index_en.html

GOOP PAGE :

http://www.ate-na.com/generator/www...888100003551_MSDS_ATE_Cylinder_Paste_US_E.pdf

(Endless, but it might help find a substitute)
 
#14 ·
Hmmm, Perhaps.

However. There was absolutely Zero rust/corrosion/damage on the passenger side, even the piston that the dust boot was destroyed and the caliper cracked, the piston looked to be in great shape, and the bore looked good. On both calipers, so I hesitate to think that the drivers side would be corroded and gunky.

I can't say for sure the fluid was dirty, as it came out like a super-soaker, I think I might have been skewed on the color, and its possible it looked good. after I got it free and could move it by hand pressure, the remaining fluid I squished out last night looked as good as the other side, like regular DOT 4 should.
 
#17 ·
Here it is, this was the worst stuck one, the others look similar or better.

You can see a bit of a ring on there, but its black, like rubber, most of it wiped off with brake cleaner and a blue shop towel, 1/2 of it remains. I **think** that's where it sat on the rubber inside seal ring, and that's residue from it of sorts. Even the dished in back of the piston was rust free, and the bore looks mint

Verdict?
 

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#18 ·
From the looks of them I would give a quick buff to the piston with a cleaner wax to pull all the fines out of the metal and then flush out the caliper and slap them back on. With liberal use of the ATE (or suitable) caliper grease and proper care and feeding they should last a while yet. Just remember to solvent wash after the wax cleaning and to grease the boots before you start slipping it all together.
 
#19 ·
Looks useable with a little cleaning to me.

C.
 
#20 ·
SO, what do you think was going on here ? Just "stiction" on the outboard pistons, and they
stuck in their position due to the rubber boot and a little grime just holding them there or what ?
They would seem to have been working properly as you drove, so was it just some quirk that they hung up, all in a similar position, and only on the outboard side, when you started to take things apart ? A little Odd to say the least, but if the outboard pistons were really stuck in that position, you'd expect corroded cylinder bores, a "tide line" in the bore as well as on the piston, poor braking performance and tell-tale signs of a stuck piston on the outside of the rotors.... none of which do I think you had, correct ? So perhaps they all (outboard) assumed the position based on the line pressure, and it's all normal- it just took a little pressure to get them to move....Or am I missing something here ?
 
#21 · (Edited)
SO, what do you think was going on here ? Just "stiction" on the outboard pistons, and they
stuck in their position due to the rubber boot and a little grime just holding them there or what ?
They would seem to have been working properly as you drove, so was it just some quirk that they hung up, all in a similar position, and only on the outboard side, when you started to take things apart ? A little Odd to say the least, but if the outboard pistons were really stuck in that position, you'd expect corroded cylinder bores, a "tide line" in the bore as well as on the piston, poor braking performance and tell-tale signs of a stuck piston on the outside of the rotors.... none of which do I think you had, correct ? So perhaps they all (outboard) assumed the position based on the line pressure, and it's all normal- it just took a little pressure to get them to move....Or am I missing something here ?
The rotors, both sides had tell-tale signs alright The outer side's were both worn right down, one side (passenger side) very bad, 14-16mm depending where you measure and the drivers side was at 19mm (See attached picture of the passenger side)

I've measured the rear's and they are good, one at ~21.5" and the other ~22" or better

As for why they stuck out? I'm still looking for that answer, and hope to find it before I put it all back together and have it happen again.

Here's what I know:

-The flex-lines are probably (99% sure) original to the truck, they still have the paint on the nuts, as did the bolts holding the rotors ect on.

-The Pistons from all the calipers looked good, even on the severely failed side that the caliper cracked and the dust boot was near non-existent, no odd corrosion or even sand/dirt/ect behind the boots

-I haven't driven the truck in much, if any salt, in deep snow, but no salt

-One of the inner pistons may have been stuck I say may have been because it retracted with the C-Clamp, but it did seem harder than it should have been, but i've never done a Unimogs calipers before, so who am I to know?

-The differential pressure light came on once before when I first got the truck, but I topped up the fluid and it was all good, obviously a sign I should have investigated further.

The truck developed a brake squeal after the light came on, I suspected wet pads, so I took the wheel off, and visually inspected, I didn't see anything, after going through some snow, it disappeared, I though maybe it was a rock, but it returned right before the failure, and I couldn't pour fluid in fast enough.

My thoughts on possible causes (I could be totally out to lunch here, I'm shooting in the dark)

-Truck has a 4 wheel park brake that applies air to the front brake system for winching, perhaps a valve got stuck slightly open with crud or similar and clamped the front brakes down a bit? But why only the outers? I had just finished winching a dodge the night before. however the indicator light works, and it was off after releasing the brakes. If that were the case, you'd think they'd come back out...since the brake system still has air.

-Collapsed flex line, most likely candidate IMO, new ones are sitting in a box waiting for the long weekend. Both sides though?

-I had to two foot brake to avoid a minivan turning right from the left lane, It was close, my winch cable rubbed her back tailgate on her van, locked up all 4 tires, its something to see on a mog on dry pavement, perhaps something happened there? It was shortly after (2-3 weeks I think) the differential pressure light came on and the master was low)

There are no other sign's of leaks that I can see anywhere, but when the light first came on it was the outermost master reservoir, and when the caliper failed on the passenger side, it was the innermost that was empty.

I assume the brakes are split into "Right front/left rear" and "Right Rear/Left front" system, I looked in the manual but it wasn't too clear.

:surrender:

Out of all this, oddly enough, I didn't feel the truck pull to one side at all, ever, nor hear grinding noise, there must have been grinding from the rotors, but I guess I couldn't hear it over the sound of the mog.

I know that rotor got eaten quick, because just before winter I had all 4 tires off, and a visual inspection was done of the pads, there was at least 60% remaining on the fronts, more on the rears, and the front pads were down to the metal, whatever it was that happened, happened recently, also the delivery inspection was done when I first got the truck and all was good then as well, little to no change in pad wear in the first 6000km that I owned the truck.
 

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#22 ·
Ok, Well that helps in terms of the information...I had ignored the big damage to the Pass side rotor, and was just concentrating on the outboard pistons- which still seem to be an odd detail to me.

The 4 wheel parking brake system might be the suspect, but, as you say : "why only the outers ?" or something like that...
The same question could be applied to the flex hose situation- why would just the outer
hoses have a problem, and why would they have the problem (apparently)
simultaneously ?

Why indeed ?

The Winch brake system, at least, is MEANT to turn everything on simultaneously, whereas you wouldn't normally expect a number of hoses to all line up and go to crap in unison, even if they were installed on the truck the same exact day on the production line...

I'm not pretending I know. I'm hoping Ben or Mark chime in and explain it all to us...
 
#23 · (Edited)
The winch park brake also has another few safety systems, it:

-Releases when the regular (rear) park brake is released (dam good thing because I accidentally hit the button in the dark once looking for another switch, at 86km/h)
and/or
-Releases when the key is turned off

Which is why its lower on my suspect list.

Also the flex hoses should have stuck inside and outside pistons out, as there is only one flex hose for each side, some have 2, mine has one per side, rest are hard lines.

Another possibility is a master cylinder or combo valve issue (I sure hope its not). I can't see it though as I had the bleeder screw removed and the piston still wouldn't go in, it was like it was plugged... Once I got it moving it was no issue.
 
#25 ·
The caliper could have been clogged on the one side. I would split the caliper and see what is going on for sure, if it ate up the rotor in that short of time. You have come this far, why stop now. Also you are able to run a brake hone inside as well, so your caliper will be just like new once the new seals are put in.
 
#26 ·
I believe this is the source of the oil migration problem and hub failures.

I was spraying WD-40 on lug nuts and got overspray on the center of the hub and it vaporized. Not believing my eyes I sprayed more WD-40 on this center part and it all vaporized. Reached down and touched it and it about burned the skin off of three fingers.

Jacked up Mog and tire would barely spin. Removed tire and brake inspection plates and brakes pads were grabbing. Pressed pads back in, lubircated and then tire would spin free.

I had to do this about 8 times before pads quit sticking.

It would get this hot in just 4 miles of driving. I am sure that if my brakes were sticking on a long trip it would have boiled out all hub oil and had total hub failure.

Drivers side hub is the one that was sticking and would only feel warm on inside of hub and super hot on outside. Other hubs would feel cool to touch.

I can tell when brake is sticking now. When coming up to stop sign, hit brakes hard and then let off then let go of steering wheel while coasting and it would pull to left. If I did not let go of steering wheel I could not tell it was pulling.

Wade
 
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