6.9 brake problems - getting desperate here....

[WGB]

I have spent a bit of time reading the workshop manual before rebuilding my rear end and I was surprised at the complexity of the 450 type anti-squat hub - there are five bearing races in total.

Not only does it have the pair of rear hub bearings that the axle runs in but it also has another pair of roller bearings and a reversed stub axle on which the hub carrier pivots in the end of the diagonal arm - but more significantly the arm that the rear calipers are bolted to sits on a large bearing that pivots around the outside of the hub carrier and is stabilised radially by the torque arm at the rear.

If there is any lateral movement in that large bearing the caliper would flap around and play havoc with the brake pedal.

While these other bearings are probably very lightly loaded compared to a rotating axle shaft they may still be affected by water or grit over the past 30+ years.

I am looking forward to my rear end re-build as there are several points to check for movement which would impact on handling and brake feel.

Bill

[s class]

Oscar, I am aware of the concern about not pushing the pedal too far.  It did happen originally in our first attempt to bleed the brakes, and was one of the reasons I thought a MC overhaul was called for.  Since then, I've been powerbleeding, so there should not be any problems with this.  It should not be a problem with the new MC in any case. 

In terms of diagnosing the problem, I wish I could be more positive.  This weekend I had another go at this brake problem.

I first disconnected the three lines off the master cylinder and bled it. (this is the new-out-the box MC). Reconnected the lines, power bled the whole system. Result - no improvement. I then decided the refit the original MC (previously overhauled, and now again stripped and inspected since I last removed it). I bled it out as recommended, then fitted it and attached the lines. I power bled again. Result - no improvement.

I then pinched off the front brake hoses, and although the pedal was much harder, it still sunk (but with more effort needed). It is possible that I didn't pinch the lines hard enough? I then pinched the rear lines, made no difference.

I inspected the pedal linkage, and it doesn't appear to be preventing the system from fully releasing - well at least on the pedal side, I don't think there is a problem. There may of course be some misadjustment problem in the booster.

So I am now still in the same situation.

I could try another known good MC from another car, but I don't want to bugger with a car that has no problems.  I could also send in the front calipers to be rebuilt again, but I just can't see how they can be the problem. 

[oscar]

It is possible that I didn't pinch the lines hard enough?

I was hessitant to throw this in my last post, I didn't want to add to the frustration but I too had the same thing happen.  Press hard enough especially with engine running and the fluid will go past the clamps.  I thought I had a eureka moment a few times with solid a pedal but I couldn't make sense of results as there was sometimes a slight giving way before the solid pedal only to discover the clamped caliper/s were closed tight.  I released clamps, repositioned in different combinations, then tightened hose clamps using a shifter and still they let fluid go by, evidenced by the closed caliper/s.  It was just another point where the situation felt helpless and I was back to square one.  It's from there I started making those hose blanks and line blanks which also left me none the wiser.

Another thing that was mentioned to me by the pros was the possibility of the firewall cracking and flexing.  I can see the logic but mine's fine as is I'm sure this car will be too.  I've wondered too about a fault within the booster.  It holds a vacuum but I can't see how it can be the cause.  When I boil down all my observations I can only conclude that the pedal sinks only whilst the calipers are connected.  It sounds the same for this car your working on.  But be damned if I know what to do when resealing doesn't work.

[koan]

Exactly the same situation I arrived at.

New and old master cylinders, pressure bleeding, rock hard with pinched off front hoses, rear made no difference.

Finger tight on my brake line clamps is not enough, they need a bit extra on the wing nuts with pliers. Given you had a concern about damage to the new hoses I suspect this is the reason there is a bit of "give" in the pedal.

Only thing I could find was the the pads moved back from the disc a bit more than I expected, especially the right side. Having a helper first press pedal and clamping while it was down, releasing and pressing again and undoing the clamps seemed to confirm this.

Don't know why the pads would pull back excessively.

I'll have a third go at fixing it one day,  with a non ATE kit.

koan

[WGB]

Only thing I could find was the the pads moved back from the disc a bit more than I expected, especially the right side. Having a helper press first press pedal and clamping while it was down, releasing and pressing again and undoing the clamps seemed to confirm this.

Don't know why the pads would pull back excessively.

As far as i know the "Back-off" is governed by the physical characteristics of the rectangular piston seal in that it seals by pressure against the edge of the seal away from the pad by twisting under pressure and then when the pressure is released the seal reverts to it's normal shape and the pad retracts slightly.

Bill

[koan]

As far as i know the "Back-off" is governed by the physical characteristics of the rectangular piston seal in that it seals by pressure against the edge of the seal away from the pad by twisting under pressure and then when the pressure is released the seal reverts to it's normal shape and the pad retracts slightly.

I understand what's supposed to happen but I did say "excessive". On other vehicles I've worked on the piston and pad motion is invisible and can be detected only by feel but on the 6.9 (mine that is) pad pull back is easy to see, amounting to about 0.5mm on side of the disc.

I removed the anti-rattle spring thinking that might be pushing the pads apart but it made no difference. There's the the anti-squeal shim I suppose but I don't see how that could be a problem.

Only things I can see possibly causing the problem is the seal or the piston finish. If the seal is over size it might grip the piston too well and not let it pass through the seal when pad wear should allow it to do, resulting in more seal distortion and hence pull back. My pistons where in good condition, I polished them up with chrome polish, is that a problem? Maybe I should have been more generous with lube when I assembled them.

The pistons needed quite a bit of force to push back in, more than I've needed on other vehicles which is another reason to suspect the seal.

As I mentioned I'd like to try again with non ATE kits and more lube.

koan

[s class]

Ok, I'm glad you people are pointing at the calipers, as that is what I had started to suspect myself.  And thanks for putting forward some plausible reasons why the calipers can in fact be at fault. 

I have seen in the past that the brake rebuilders I use polish up the pistons to a much more abraided finish than I would have thought correct, but maybe they know something I don't.  I think I may send in the front calipers to them to have a crack at.  Its not expensive (less than $100 for both fronts). 

[koan]

I have seen in the past that the brake rebuilders I use polish up the pistons to a much more abraided finish than I would have thought correct, but maybe they know something I don't.

Is it possible a dull finish from 400 or 600 grit wet and dry leaves fine scratches that are not deep enough to leak fluid but still hold fluid providing lubrication to piston allowing it to slide in the seal?

koan

[WGB]

Isn't that what a cylinder hone does?

Bill

[koan]

Isn't that what a cylinder hone does?

Yes but I've always thought that was to break (brake?) the rings in, are you suggesting it's to hold oil?

koan

[kastad]

I'm a novice in brake fixing but I had some similar problems in a Volvo, and it was equipped with a vacuum brake servo unit with bad gaskets. It made the brake fluid go directly into the inlet manifold. I don't know how the brake servo unit in the w116 works, but can this be the reason for your problems?

[koan]

I'm a novice in brake fixing but I had some similar problems in a Volvo, and it was equipped with a vacuum brake servo unit with bad gaskets. It made the brake fluid go directly into the inlet manifold. I don't know how the brake servo unit in the w116 works, but can this be the reason for your problems?

Certainly not in my case, my car has had spongy brakes for at least two years, if fluid was being sucked out via the booster it would have long since disappeared.

koan

[TJ 450]

Isn't that what a cylinder hone does?

Yes but I've always thought that was to break (brake?) the rings in, are you suggesting it's to hold oil?

koan

Both are correct, retaining oil is the long term function.

Tim

[koan]

WGB: Isn't that what a cylinder hone does?

ME: Yes but I've always thought that was to break (brake?) the rings in, are you suggesting it's to hold oil?

TJ :Both are correct, retaining oil is the long term function.

I'll get the emery paper out then but I might talk to someone first.

koan

[Papalangi]

"Maybe I should have been more generous with lube when I assembled them."

I learned that only brake fluid should be used during reassembly.  And that petroleum products can damage the seals. I always thought it would make the piston slide too easily in the bore and could lead to it retracting too far.

While looking for backup before posting "no, no, never use lube" I did some searching and found that ATE sells a silicone caliper lube that several forums say should be used in the bore and on the piston.  I've also found that Honda sells OEM rebuild kits with what it calls "red rubber grease".

Now I'm confused.

Michael