Binding Rear Brakes - not the usual

[jfheath]

A fellow member reached out to me by PM with a binding rear brake problem, and we have spent a few happy hours on the phone, and he has spent many more not-so-happy hours in his garage trying to pin this down.

He had previously had the calipers apart and checked pistons, cleaned grooves, fitted new seals.

We have made progress, but the solution is not definite yet - particularly since it sounds as though there are two completely different issues.

The bike is a UK 04 ST1300.

We did the usual tests with the SMC applying the SMC while turning the rear wheel. SMC pumped up the rear outer pistons, wheel locked and never released.
That doesn't necessarily meant that the SMC is bad - it could mean a few other things as well, but the crunch came when the front leeft centre piston bleed valve was opened and the rear pistons could not be pushed in. But when the outlet port of the SMC was cracked open the reear outer pistons could be pushed in by hand.

So - new SMC ordered and fitted. And that test was repeated with success.

But the rear wheel is still only getting beteen half and one turn - with effort, after the brake pedal has been pumped up.

So we got it so that the wheel was locked (I say 'we', I'm on the phone while he is on speaker phone getting his hands dirty at the other end of the country).
We did the previous checks, and fluid is getting through the SMC from behind to be ejected through the front left centre bleed valve.
The front left centre piston can be pushed in by hand - so that confirms the rear master cyclinder comepnsation port is clear.

So with rear wheel locked, we opened the bleed valves to release the pressure. Nothing came out. The fluid is not under pressure. Therefore it is not fluid pressure that is locking the rear brakes.

So we have concluded that the problem is nothing to do with pressure build up, but is a mecahnical issue.

The caliper without pads is sliding smoothly on the slider pins all the way in and all of the way out.
There are no marks on the rear stopper bolt.
There was no rifling in stopper bolt hole
Pads are brand new OEM, brand new pad springs, brand new chromed retainer clips, brand new pad pins with rubber grommit.

When the pad chrome retainer clip was fitted , it felt slack - so I guess there is some wear in the recess to accomodate the clip.
The owner had removed the slider pin in order to remove the caliper.
There was no binding in the movemenet of the caliper towards and away from the wheel.

We are thinking that somewhere along the line, something has not been fitted correctly and this has resulted in deformation of the caliper (unlikely I think), or the caliper bracket. We think that when the pedal is pumped up in order to meet with any resistance, the action is forcing something to sprain - into a position from which it cannot release. Spinning the wheel does enable the pads to release a bit - but not enough. He has verified that the brake disc (rotor) is not warped.

He is going to try to observe what happens at the caliper when the pedal is slowly pumped. But we are suspecting that the rear brake caliper bracket is bent slightly.
At least we are hoping that is the case rather than the slider pins being cross threaded or bent (the caliper moved back and forth smoothly).

Its easy for me to suggest this - but it is £200, and it still may not solve the problem if it is the pins.

Am I missing something obvious ??

 

[Sadlsor]

Has he sent you any photos or video?
As poor a mechanic as I am, and being a visual learner, one or both may prove valuable to the effort.

 

[jfheath]

No. I haven't had the need for any so far.

 

[Igofar]

Possible Flag....
The owner had removed the slider pin in order to remove the caliper.
He may have either cross threaded the steel pin back into the soft aluminum caliper....
Or....
He tightened the pin with too much force and sunk it into the aluminum, thus tilting the pin etc.

 

[Andrew Shadow]

You seemed to have thoroughly gone through everything to do with the braking system John, so I have nothing to offer regarding that.

What about the rear wheel? Is there any possibility that any part of the wheel installation was not carried out properly and is causing a problem?

 

[CruSTy]

Possible Flag....
The owner had removed the slider pin in order to remove the caliper.
He may have either cross threaded the steel pin back into the soft aluminum caliper....
Or....
He tightened the pin with too much force and sunk it into the aluminum, thus tilting the pin etc.

Seen that before!,

 

[jfheath]

Possible Flag....
The owner had removed the slider pin in order to remove the caliper.
He may have either cross threaded the steel pin back into the soft aluminum caliper....
Or....
He tightened the pin with too much force and sunk it into the aluminum, thus tilting the pin etc.

Yes. I did advise against removing the pins at all - the caliper slides off the bracket one the bracket has been removed. But that is one of the things he is looking at now. That possibility of over tightening the slider pin is greatly increased by the fact that the early manuals show a diagram with a torque setting of 69Nm assigned to that pin - about 3 times what it should be. It appears that the pin has been mistaken for the stopper bolt. I alerted him to this in our first conversation when he told me that he had taken out that pin.

I feel that he is going to need a new bracket, but he's the only one that can make that decision.

You seemed to have thoroughly gone through everything to do with the braking system John, so I have nothing to offer regarding that.
What about the rear wheel? Is there any possibility that any part of the wheel installation was not carried out properly and is causing a problem?

The rear wheel seems to be ok. I had him spin the rear wheel when he had the axle untightened, and then spin it again with the axle torqued up. Both ran freely. So there isn't a bearing installation issue which side loads the bearing when it is clamped up. He says the bearings do not feel rough or notchy.
He normally puts the stopper bolt in before tightening the axle - because he challenged my request to tighten the axle. I can't think of anything else to check.

Actually. I've just thought of one thing. He mentioned last night that his exhausts were in the way for something he was doing. I need to ask how he gets the stopper bolt out and how he checks that the pads are properly located in the retaining clip - because I cannot do either of those unless the left exhaust is swung down.

 

[Oldbikefixr]

Just a fleeting thought. Does his front wheel spin with no effort?

 

[Obo]

Is the rear disk warped? That could explain the half to one turn before things get "sticky."

Check the run out with a dial caliper (at best) or a static device at worst (like a metal coat hanger on a bit of wood sitting on the floor and just touching the side of the caliper)

 

[jfheath]

Thanks for these two suggestions @Oldbikefixr and @Obo (as well as @Igofar and @Andrew Shadow before them).

Becasue it is an interesting problem, I'm going to keep adding my comments so that anyone reading int he future gets a full picture of hwo this was diagnosed as I (we) diagnose it. As yet we don't have the full answer, but I think I know what the full answer is. (At least I can guess, but I'm not footing the bill).

Just a fleeting thought. Does his front wheel spin with no effort?

Now that is an excellent suggestion that I doubt that many people would think about immediately, and one which caught me out when I got my first ST1100 with the combined braking system. But since that one occasion, my first reaction to hearing that someone's back brake is sticking is always to head fro the front brake. Have they put rear pads in the front, have they put heat shields on the (new) front pads, have they put the rear retainer clip in the front caliper bracket. All of which will make the front brakes bind, activate a perfectly good SMC, and make the rear brakes very hot.

Is the rear disk warped? That could explain the half to one turn before things get "sticky."
Check the run out with a dial caliper (at best) or a static device at worst (like a metal coat hanger on a bit of wood sitting on the floor and just touching the side of the caliper)

Another excellent suggestion, but, no I got him to check that early on. The symptoms that he described:

• The back brakes were dragging badly and after a short ride they came back hot. (Incidentally, the comment was that he went for a ride and only used the front brakes, but the rear was hot), which is a misunderstanding of how the ST1300 brakes work.
• The brakes do back off a bit in the garage if he forces the wheel to rotate. After that, he always had to pump up the rear brake with the pedal.

These two observations are at odds with each other - which suggested two problems to me. But it was important to identify whether or not the SMC was at fault. He couldn't get fluid to push back from the rear pistons to the bleed valve on the front center piston (which is directly connected to the SMC inlet port.) But he could get the fluid to emerge if he cracked open the banjo bolt on the SMC outlet port.

So that clearly identified the SMC. Which has been replaced now. The SMC is functioning. There is no fluid pressure to release when the back brake is binding. The SMC is doing its job properly relealsing the pressure back to the reservoir. (Which has plenty of air space).

So yes indeed the situation could be caused by a warped disc. With old pads and a properly moving caliper, you cannot feel a disc warp when braking. The caliper takes up the movement, and doesn't pass any feel to the pedal. Not so if you fit new pads. The caliper has no room to move with the disc warp, so the pistons pulse and you feel it under your foot.
But that movement when the brake isn't applied nudges the pistons into their bores, so every time you press the brake pedal , the first press is taken up by pumping the pistons back out again.

But he has checked the disk for warp and it is running true. I don't know how he tested it.

But that behaviour makes me think that when he applies the brakes the pistons are forcing the caliper to slide along pins which aren't parallel, so thaey are being sprung apart or together and keeping the caliperfrom returning. I also suspect that the pads may be not parallel to the disc surface for some reason - so that when the brakes are applied, the pads are forcing the caliper to rotate slightly - again jamming the sliding mechanism in place. We know that can happen if the pad retaining clip seat is worn. It results in the rifling appearance that Larry has described which he spent a long time testing and trying to diagnose. If I remember correctly, in that case the brakes were moaning as well. Since then, I know that Larry has become familiar with the signs of a damaged / distorted / worn caliper bracket and has replaced many since - all with excellent results.

One of the last things I found out during last nights call was that he is removing the slider pin to remove the calipers. I don't know why. And I don't know how he knows that the lower slider pin is in place. I had recommended that he didn't - but perhaps that is the way to get the caliper off without removing the pads ? I don't know. I always assemble bracket and caliper. Put the assembly over the disc/rotor and slide the axle through. Fit the pads last. Which is what I had previously described to him as 'the way to do it'. Leave the slider pins in place. Nothing good will come of removing them. So I don't know how he makes sure that the pads are located properly behind those tags on the pad spring. I need to find out about that before diagnosing duff bracket.

Interesting stuff.

 

[Andrew Shadow]

As I wrote earlier you, are covering all of the probable things that one suspects in these situations so I don't see the point in repeating them.
So, what about the stupid stuff?

Is the reservoir overfilled?
I know that you stated that the calipers were rebuilt and that the SMC was replaced so the system was bled. You also wrote that new pads have been installed but provided no chronology on what was done when and in what order. Is there any possibility that the work to the hydraulic side of the system was done with the old pads fitted and sometime thereafter the new pads were fitted and rechecking the reservoir level was never thought of?

What about hydro-static lock?
All the calipers were rebuilt and all new seals were installed. Is there any chance that he got carried away with lubricating things and that there is an excess of lubricant in the slide pin bores that is pushing the caliper away from the rotor?

Not earth shattering suggestions I know, but you have covered all of the logical and probable possibilities. Some consideration of the improbable can't hurt and is not time consuming before proceeding with ordering more parts such as a new bracket.

 

[v8-7]

had a truck that had a binding front brake. Loosened the bleed screw and no fluid came out, but enabled me to push the piston back in.
Turned out to be a brake line that was swollen on the inside .
Pushing the bake pedal generated enough pressure to force the fluid through, but the bad brake line acted like a check valve and did not allow the release of pressure.

 

[RobbieAG]

It may be time to get a used caliper and bracket off eBay to rule that out.

 

[jfheath]

Stupid suggestions are the best ones @Andrew Shadow . But no, the reservoir had plenty of air space, I checked the first day, and again yesterday.

We talked about the slide pin bores, and he cleaned them out while he was on the phone. Yes, it had been an issue and movement improved after. But it didn't cure the sticking problem. He mentioned that there seems to be a relief mechanism on the seal. He saw the compressed air pop out as he slipped the slider pin inside. Ive never noticed that. I need to take a look. He described it as a lemon shaped lip.

@Andrew Shadow
Not earth shattering? On the contrary - very valid suggestions. I have had both situations - binding back brakes after a service, one day before a trip to Spain. The only thing I found was too much grease and I watched the caliper slide back by itself after I pulled it across away from the disc. Overfilled reservoir was the cause of my ABS system failing on my 1100. The modulators couldn't perform their initial cycle, and blew the fuse. But yes, we covered both of those. I do the stupid suggestions first, cos they are easy to miss, and cheap fixes. And better than that, if it turns out to be the answer, they think you're a genious !!

@v8-7 . That is an intriguing comment. when you were able to push the pistons back in, was that because the bleed valve was loosened, or were you able to do it when the bleed valve was closed ?

A 2nd hand caliper / bracket is a cheaper solution, but its pot luck as to whether or not it will have similar problems. I landed on a front right caliper when the thread in my bleed valve hole got damaged somehow. It was in better condition than the one I took off. That would be a suggestion that I can make.

Saturday tomorrow. I reckon I'll get a call !

 

[SMSW]

A 2nd hand caliper / bracket is a cheaper solution, but its pot luck as to whether or not it will have similar problems. I landed on a front right caliper when the thread in my bleed valve hole got damaged somehow. It was in better condition than the one I took off. That would be a suggestion that I can make.

What are the chances that two caliper/bracket assemblies have exactly the same problem when this certainly seems to be a one off, i.e. rare conundrum? I'd think a used bracket that has no visible defects - chatter marks, wear in the pad pockets, etc. would be eminently usable. It would really come down to how much money is being saved.

 

[Igofar]

....He saw the compressed air pop out as he slipped the slider pin inside. ...
He is probably using way too much grease on the pin, causing suction, which is not allowing it to move freely.
Tell him to completely clean out all the boots/grommets with an ear bud (q-tips to the folks over here) and clean all the grease off everything, then only leave a very slight amount (think film) of grease on the sliders etc.
Have him press them in/out a couple times, then make sure the boot didn't come out of the groove that it sits in.
Thanks for trying to help him John.

 

[Ryan_B]

Pads are brand new OEM, brand new pad springs, brand new chromed retainer clips, brand new pad pins with rubber grommit.

You might have him verify the correct pads were actually sent. I certainly believe he ordered the correct parts, but sometimes mistakes happen on picking parts. If the old ones are still handy you can compare them back to back.

Good luck! You will track it down.
Ryan

 

[v8-7]

when you were able to push the pistons back in, was that because the bleed valve was loosened, or were you able to do it when the bleed valve was closed ?

Yes, fluid came through the open bleed valve, but only when I pushing the piston back in . . the fluid was blocked from traveling back to the reservoir .

 

[jfheath]

The grease ? Yes there was too much grease in there, Larry. I could hear it over the phone the other day. He cleaned it all out and it improved the movement but it didn't cure the sticking problem.

Thanks @v8-7. With the new SMC the fluid is now going back to the reservoir when pushing the pistons in. I need to check that with him though. We checked the fluid return to the reservoir from the front left centre. We checked the fluid return to the front left centre. He was going to check that the pistons pushed in with everything closed up after we put the phone down.

How did you discover that it was a swollen inside the tube ? is it something you can see or feel ? I wonder if he has been using hose clamps on his brake lines ? Hmmm. I will ponder on that as a possibility.

@Ryan_B . Good thought about checking the new pads. That is something that I would do automatically in the garage without thinking about it. But it never occurred to me when talking over the phone. A Different part of the brain in action.

He has some old OEM pads too. I think I'll ask him to try fitting those. If there is a problem with the slider pins, the worn down pads will put the pistons and caliper in a different position relative to each other. It might reveal something.

I need photos I think.

 

[v8-7]

The fact that loosening the bleeder allowed for normal piston retraction meant there was blockage .
This was a 1970's truck and there wasn't much else it could be, or I got lucky and my first guess worked..