Brake problem

[koan]

I'm thinking maybe the rubber dust seals are stiff and not deforming like they should do, which is maybe producing producing a retarding force withdrawing the pistons back further than they need to go.

Hows that sound?  Plausible?

Very possible, the seals seem to be crammed in there.

koan.

[TJ 450]

I seem to recall reading somewhere (perhaps in the kit manual) that only brake fluid should be used as a lubricant when inserting the pistons and seals, and that would make sense.

I used PBR rubber grease when I did mine and I didn't use it sparingly either. I think it's more than a coincidence that my brakes are also behaving strangely.

Furthermore, some food for thought.  There's a bit of grease in the dust seals as well.  I greased everything with a smear - all seals and metal parts, with that red rubber grease, nothing with brake fluid.  But there's clearly too much behind the dust seals.  Maybe that area should remain dry. Maybe that volume of grease is supposed to be air because as the pistons extend, this area behind the dust seal is probably air tight and forms a bigger vacuum that causes the pistons to be pulled in as the pedal is released.

I think what you're saying here is spot on, also the grease will ensure that no further air will be intoduced to the chamber behind the dust boot meaning that once the initial air has been expelled, the vacuum will be even greater still.

Tim

[koan]

Actually, I just went and picked the dust seals and heat shields off the front right caliper, put a pair of pads in and voila.  You're right 13B. 

I've not quite got right the fitment of the dust seal a few months ago when I rekitted these calipers.  I remember scratching my head over how it fits into the outer groove of the piston but it was too hard to describe or take a photo of so I left it and fitted them to how I thought they looked right. 

That's a good find oscar and 13B, wouldn't have thought the dust boots could be the cause.

I used ATE kits with instructions and couldn't make sense of them but like you I did what seemed right. There's a groove in a piston and a seal with a hole in it, not much to get wrong. Can't remember exactly what the difficulty was though.

So have I/We missed something?

Maybe when the pads wear down a bit it will improve but mine haven't yet.

Doubt vacuum would be the doing the pull back but anything is possible.

koan

[koan]

I seem to recall reading somewhere (perhaps in the kit manual) that only brake fluid should be used as a lubricant when inserting the pistons and seals, and that would make sense.

I used PBR rubber grease when I did mine and I didn't use it sparingly either. I think it's more than a coincidence that my brakes are also behaving strangely.

I used brake fluid not grease, my brakes are behaving strangely too. Not convinced about the vacuum idea.

Just been out looking for the instruction sheet, thought it might be around somewhere but no.

Did it say to put the boot into the caliper first and push the piston through it?

koan

[TJ 450]

I threw out the instruction sheets, but in the Haynes manual, it states to press in the pistons then fit the dust boots using brake (hydraulic) fluid as lubricant.

I used that method. Edit: Using PBR rubber grease instead of brake fluid.

Tim

[oscar]

I can't find the instruction sheets either and the bendix kits I have don't have instructions. I think I'll try a set of bendix calipers now. 

Maybe when the pads wear down a bit it will improve but mine haven't yet.

Doubt vacuum would be the doing the pull back but anything is possible.

The vacuum idea sounds good but I too have doubts now.  Main reason because of when I packed between the pistons and disc with pads and metal which restricted the amount of outward piston movement that would otherwise create that vacuum.  True the pistons weren't all the way in to a stop but it proved that no matter how new or worn pads and rotors are there was still enough retraction to cause this problem.  With pistons pushed in like I did you would expect there to be very little if any vacuum as there might be for worn pads and rotors.  The only thing that seems to ring true is that the absence of the dust cover eliminates the problem, it's still hard to believe it's purely a mechanical action of the dust seal that pulls the piston back in regardless if the piston is protruding as with worn pads and rotors, or, if the piston is in a withdrawn position as if new pads and rotors were installed.  Anyway, other calipers and rekits are on the menu.  Including testing some other calipers as is.

[13B]

I'm prepared to be flamed severely here but since all the dust boots do is keep dirt from around the pistons over a very high milage (its common for brake calipers to cover 300,000km or more before being rebuilt in many cars) and that Povos environment covers a relatively short distance, why not just leave the dust boots and squeal shims out?  The car will be lucky to cover 1000km per year.

I.

[s class]

If you have a dust seal to sacrifice, try making a small hole in it, then install it.  This way you will discover if the problem is due to the cover's mechanical tension pulling back, or if its a vacuum issue. 

[Papalangi]

Morning guys, I'm a bit late chiming in but I think you will find that the seals in the front calipers are causing the problem.  Three or four quick very pumps on the pedal and then holding it down should give you a rock hard pedal since you've pushed the pads out and  refilled the master before the pads can retract.

The seals are designed to twist when the piston is pushed out and untwist just a bit to pull the pad a fraction back from the disc.  As the pads wear, the seal will creep along the bore but should not slide otherwise.  If the caliper bore is to big or the seals are incorretly lubed, they will slide rather than twist.  This means that any spring back in the pads will push the piston in farther than it should since the resistance of the seals is too easily overcome.

I did find this which also mentions that rotor wobble can push the pads back and make it spongy.

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/disc-brake2.htm

Michael

[oscar]

I'm prepared to be flamed severely here but since all the dust boots do is keep dirt from around the pistons over a very high milage (its common for brake calipers to cover 300,000km or more before being rebuilt in many cars) and that Povos environment covers a relatively short distance, why not just leave the dust boots and squeal shims out?  The car will be lucky to cover 1000km per year.

I've thought the same thing.  Particularly for this car, it's really not going to matter if the dust seals are removed.  At this point, whatever gets me firmer brakes I'll be happy with.

I did a search the other night and talk about a recurring theme, for a w116 no matter which model, there's quite a few mentions of repeated bleeds, new components and little or no improvement.  Makes me wonder what is achievable for w116 front brakes.

If you have a dust seal to sacrifice, try making a small hole in it, then install it.  This way you will discover if the problem is due to the cover's mechanical tension pulling back, or if its a vacuum issue. 

Brilliant idea sclass.  So simple.  Two nights ago I read this and went to try it out at midnight after I got back from work.  I went to the LHS and put two decent holes rather than pin pricks to be doubly sure a vacuum couldn't be produced, plus it didn't matter if I sacrificed these seals if I was going to buy more seal kits.  The result was more than disappointing. 

Firstly, no change with holes in the seals. I thought there might have been but measuring for firmness by feel is so subjective.  I thought it improved at first but I don't think it did.  So I was expecting that removing these seals they'd firm right up like the RHS.  Not so, it's still spongy (just with the LHS callipers attached).  I feel like I'm back to square one. 

Only thing to do now is bench these ATEs and try a different set of used callipers plus rebuild a set of Bendix ones which are partly disassembled already.

Morning guys, I'm a bit late chiming in but I think you will find that the seals in the front calipers are causing the problem.  Three or four quick very pumps on the pedal and then holding it down should give you a rock hard pedal since you've pushed the pads out and  refilled the master before the pads can retract.

The seals are designed to twist when the piston is pushed out and untwist just a bit to pull the pad a fraction back from the disc.  As the pads wear, the seal will creep along the bore but should not slide otherwise.  If the caliper bore is to big or the seals are incorretly lubed, they will slide rather than twist.  This means that any spring back in the pads will push the piston in farther than it should since the resistance of the seals is too easily overcome.

I did find this which also mentions that rotor wobble can push the pads back and make it spongy.

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/disc-brake2.htm

Hey Papa, I'm having trouble believing theat these seals twist.  They just seem very uniform without folds or diagonals in the expanding part.  For old callipers it does make me wonder if my bores are worn perhaps but the things are so darn tight still.  Every time I've pushed the pistons in I've had to use a long screw driver to get leverarge to make the piston a pivot point to push them in.  Granted they're imposible to push in if they become skewed but I can't push them in by hand.

[Papalangi]

We're talking a few thousandths of an inch travel between relaxed and under pressure.  It would be hard to see.  The dust seal is just along for the ride, it's the o-ring that brings the piston back.

The fact that you have to lever the pistons back in does argue for the fact that the bores are not too big.

Michael

[koan]

I did a search the other night and talk about a recurring theme, for a w116 no matter which model, there's quite a few mentions of repeated bleeds, new components and little or no improvement.

That's the impression I've got as well. You might be lucky, do a rebuild and end up with a firm pedal but often it ends like what we have.

Hey Papa, I'm having trouble believing theat these seals twist.  They just seem very uniform without folds or diagonals in the expanding part.  For old callipers it does make me wonder if my bores are worn perhaps but the things are so darn tight still.  Every time I've pushed the pistons in I've had to use a long screw driver to get leverarge to make the piston a pivot point to push them in.  Granted they're imposible to push in if they become skewed but I can't push them in by hand.

I'd go along with papalangi except twist is the wrong word. The seals flex, change from a square section to being pushed out around the piston, rhombus is the correct term I think. The distortion is intended and it is what gives the running pad to disc clearance when the seal returns to its square shape pulling the piston back. I've always thought it to be a minute amount not approaching the millimeter we see.

Have a read of this http://www.autoshop101.com/forms/brake04.pdf, especially page 8.

koan

[oscar]

Ahh I see!!  Sorry papa, I orignally thought you meant the dust seal but now I get it.  Koan, that page 8 shows it perfectly.  It too says "twist" as well which had me thinking turning twisting motion so I agree that flex or "deform" which that link also says, is perhaps a better term.  Looking at the left diagram on page 8 of the non deformed square seal it looks more trapezoid? than square.  I'm sure the ATE seals were perfectly square but it made me wonder if there was a right and wrong way for these seals to be inserted. 

FWIW, I was back in the 350 today which has ATE callipers too and whilst the engine is running and without pumping, the brakes come up firm after 2-2.5 inches of brake pedal depression and limit of movement was under 3 inches, measured at the base of the pedal.  7cm was the total range of movement. They're pretty good and I've only ever replaced front hoses.   Not sure if the callipers have ever been serviced but clearly, povo's are woeful in comparison.  Improvement should be attainable and expected. (just pep talking myself up )

[s class]

Over the years I've done various rebuild work at various times on my 280SE's braking system, and its always come up pretty good (not super impressive) and I've never had any trouble bleeding it.  Trusty rusty gave me the same results and same feel when I rebuild its braking system (new disks, pads, hoses, M/C, rebuilt calipers).  The fact that the two cars had the same braking feel encouraged me to believe that's as good as it gets. 

Now the R107 on the other hand, I've put in new hoses, new M/C and rebuilt the calipers with genunine MB supplied seal kits.  I've bled the system out like 8 times over the years and it never gets better than just plain disappointing. 

the latest episode in my 70's benz braking experience is a customers W123 that came to me last weekend.  The car had just had its braking sysem rebuilt by a specialist benz workshop.  I am stunned at the braking performance.  the braking is so hard, and so sharp, that its tricky to drive - its like some new cars that you literally just caress the brake pedal with your foot, and there is a vicious retardation force.  I really need to find out what was done, and what the magic ingredient is. 

The only potential difference between this w123 and my cars is that the W123 is a diesel with a vacuum pump in good condition - my cars are gas engined with varying qualities of vacuum - I know that the R107 is particularly challenged in this regard. 

[koan]

I'm sure the ATE seals were perfectly square but it made me wonder if there was a right and wrong way for these seals to be inserted. 

Don't think so, either way round they are symmetrical.

But I worry I may have turned one 90 degrees (oscar twist) as I pushed it down the bore.

The car had just had its braking sysem rebuilt by a specialist benz workshop.  I am stunned at the braking performance.  the braking is so hard, and so sharp,

Was the pedal hard?

By the sound of it there was very little pedal movement and that's what I expected after a rebuild. Some cars I've driven, the pedal goes down a small distance, taking up the slack and then harder breaking is more force on the pedal without any noticeable further travel.

Don't think vacuum is my problem, there's plenty of assist and the car stops with not much force on the pedal, it just goes down along way. The braking is progressive, more force, more stopping.

koan