6.9 brake problems - getting desperate here....

[s class]

OK, so I'm finally done with the dark blue Webasto-pre-heater 6.9 project car..... but I can't get the brakes right!

Originally, I had removed the front calipers to do the suspension work.  On recommissioning, I replaced all 4 brake hoses as they were looking crusty (very, in fact).  I also put seal kits in the front calipers.  I found that I couldn't get a decent brake feel, as the pedal would sink down in the traditional master cylinder with internal leakage manner.  I bled repeatedly and thoroughly, in an effort to ensure it wasn't trapped air.  Eventually I concluded that it must in fact be the master cylinder that was bad, so I sent it out for reconditioning.  When it came back, I installed it and found that the scenario was exactly the same.  I again bled thoroughly, and eventually gave up.  Yesterday I installed a brand new master cylinder I had on hand.  After thorough bleeding, when testing for pedal feel, the situation seemed improved somewhat, but not to the point that I was happy.  During test drives, at higher speeds, the braking seems fine, but when sitting at idle, the pedal still sinks down, and one must pump the pedal.   What is going on here!!

I have checked and re-checked, and I can't see external leaks anywhere on the system.  I would like to think I can rule out the master cylinder now.  I see no reason to doubt the front calipers.... they seem to be behaving correctly.  When the front of the car is lifted off the ground, I can turn the front wheels freely, and just hear the faintest sound of the pads brushing on the discs, both sides the same. 

I've never struggled like this on a W116 before. 

[alabbasi]

I've found that these cars don't like vacuum bleeders (at least not mine). I bought a pressure bleeder which seems to do a great job. You can try that (or two man approach).
Other then that and if your 100% sure that your master cylinder is good (both may be bad), then check or replace your brake hoses. If they are old, they may look good externally but could swell under pressure which could result in what you're seeing.

Good luck!

[s class]

Thanks Al,  4 new genuine MB brake hoses here, and I'm power bleeding.  A couple of times I've also checked with the traditional two-man approach. 

[alabbasi]

Thanks Al,  4 new genuine MB brake hoses here, and I'm power bleeding.  A couple of times I've also checked with the traditional two-man approach. 

Are there any leaks (wetness) from the calipers or anywhere on the brake lines (which could rust). If not, then it has to be the master cylinder.

[s class]

Al, no wetness that I can see.  Logic tells me that it must be the master cylinder, but with three tries :

a) original unit before overhaul
b) original unit after overhaul
c) new unit

all giving the same result, its hard to believe. 

FWIW, the sinking pedal problem seems to become more apparent as engine bay / brake temp is up to normal operating conditions. 

I think my plan for tomorrow is to disconnect the three brake lines at the master cylinder, and plug them with blanks.  I will then try to bleed the master cylinder, and it should then be obvious if its at fault or not.  I will then reconnect the three circuits, one at a time and repeat until I can locate the problem.  It will just take a lot of hours and a lot of brake fluid. 

[peterq]

If the rear calipers have been replaced in the last few years they *may* be the wrong ones - for 280-350 models. In that case the bleed nipple ends up at the bottom rather than the top and it is nearly impossible to get the air out. Apparently the correct calipers are NLA and you get sent the others, usually without warning. Either remove them from the hub and bleed them upside down with a bit of wood between the pads, or swap sides and get longer lines (those for a BMW of some sort fit).

[craigb]

Maybe do a search on Oscars posts of brake problems - sounds very familiar! Did you ever sort that?

My other suggestion was similar to the rear caliper one - if the fronts have been off and rekitted on the bench they could be bolted back on on the wrong sides - which puts the bleed nipples at the bottom and therefore leaving a pocket for air at the top.

But I reckon Oscar and maybe Koan had issues to do with the pedal pumping up because they had assembled with rubber grease and maybe some pad knock off from worn discs - pushing the pistons back and needing that travel to push them back in again...........? something like that but left me with the thought that I would just use brake fluid on assembly and not grease.

I'm sure after all the trauma one of these will respond.....with passion!

[s class]

Thanks guys.

I am aware of the different rear calipers, and these are indeed the correct ones, correctly installed.

I did not split the front calipers.  I only removed them, cleaned them and installed new seals with brake fluid as lube.  No grease anywhere. 

My understanding is that there are only 3 possible causes for my miseries :

a) air trapped somewhere
b) master cylinder passing fluid internally
c) external leak somewhere.

OR - is there some other possibility I've missed?

[KenM]

Can't offer any practical advice other than I think you have the right approach with blanking off the ports and then trying them one at a time, a methodical process of elimination, element by element is the only way to be sure of finding and fixing the problem. Can be most frustrating. Sounds like a curly one though, all the best.

[TJ 450]

I have found that every brake job I've done on these cars has left me with a spongy pedal that almost goes to the floor... I can't complain about my 6.9's brakes now though. After a few thousand K's, the brakes are very smooth and responsive allowing me to stop promptly in panic situations and lock up all four wheels at will.

Tim 

[wbrian63]

I left a reply over at M-100. I think your problem is still the MC. Check over there for further suggestions.

Regards

[oscar]

Maybe do a search on Oscars posts of brake problems - sounds very familiar! Did you ever sort that?

No, unfortunately.  I've been keeping an eye on this thread because you're right, it sounds exactly like what I've got and after extensive fault finding and exchanging components I'm none the wiser. 

Just to summarise,
- I made plugs for the MC.  Super solid pedal with and without engine running.
- I plugged the ends of brake hoses.  Still solid pedal with and without engine running.
- I bled and bled anytime I cracked the system, plus ended up powerbleeding with a homemade unit using a pressurised weed spray cannister like craigb suggested to no avail.

3 plus 1 new MC, two sets of brake hoses, resealed callipers.... the only things not replaced were the booster and steel lines.  I came to the conclusion that nothing made sense and I gave up.  The pedal still feels soft.  The wierd thing is the pedal slowly sinks when reasonable force is applied to the pedal whilst the engine is running which would lead one to think the MC was leaking internally, yet blanking tests proved otherwise.  The last time POVO went out was in November which saw a brake binding issue and top speed around the track went from 160 to 140kph.  So one would think air is expanding in the lines/calipers.  No fluid loss, I did do a quick two man bleed a couple of months ago and air came out from both rears, fronts ok.  Still soft pedal two weeks ago.  Unless my resealed calipers are sucking air in during pedal return I'm really clueless, at a loss to explain, and POVO mojo is non existant.  My 350's brakes are tops and as tempting as it has been to swap it's calipers over, I'm reluctant to do so in case the brake curse infects the 350.  I do hope you find a solution sclass.  One suggestion from googleing was internal corrosion in brake lines. But IIRC you had new lines made for this car??  I was tempted to try the same, make my own, but the flaring tool to do merc style connections cost too much.

[s class]

Thanks Oscar.

No, this is not my car, its a fellow club-member's car.  I did not fit new steel brake lines.  But you raise an interesting point.  Suppose the lines have rusted inside, how would this actually affect the pedal feel?

Last night I got under the car, and really checked thoroughly for external leaks.  It seems 100% clean.  I bled again, and got zero air out, so not surprisingly, the result is the same still. 

This morning I took the original MC (the one that has been overhauled) back to the brake place.  THey stripped it, found it to be perfect inside, reassembled anbd bench tested it.  It is apparently perfect.... oh and I do trust these guys, they aren't just trying to cover their backsides...

[Big_Richard]

I have found that every brake job I've done on these cars has left me with a spongy pedal that almost goes to the floor... I can't complain about my 6.9's brakes now though. After a few thousand K's, the brakes are very smooth and responsive allowing me to stop promptly in panic situations and lock up all four wheels at will.

Tim 

why is it that mine has never had any problems what so ever?

i rebuilt my rear calipers myself, i have 4 new disks, i have new ate front calipers. i have plastic coated stainless steel brake hoses. master cylinder is original with a new tank + grommets.

before and after my brakes were perfect. now they just actuate more forcefully and effectively and there is zero pedal movement once they lock.

[s class]

I have found that every brake job I've done on these cars has left me with a spongy pedal that almost goes to the floor... I can't complain about my 6.9's brakes now though. After a few thousand K's, the brakes are very smooth and responsive allowing me to stop promptly in panic situations and lock up all four wheels at will.

Tim 

why is it that mine has never had any problems what so ever?

i rebuilt my rear calipers myself, i have 4 new disks, i have new ate front calipers. i have plastic coated stainless steel brake hoses. master cylinder is original with a new tank + grommets.

before and after my brakes were perfect. now they just actuate more forcefully and effectively and there is zero pedal movement once they lock.

That's how my 280SE and trusty rusty are.  And I've never had any grief with either of them trying to get it right.  My 450SL gave ma lot of curry, and its still a little spongy, though there is no sinking pedal symptom.