BMW 2002 FAQ - Slight brake drag - the continuing saga

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Thread Topic: Slight brake drag - the continuing saga

Date: 7-16-08 07:14
From: RudyB
Subject: Slight brake drag - the continuing saga

As noted in the subject heading, one of my 02's is experiencing a very slight brake drag and this is slightly greater than normal.

I tried to describe the issue in another post and acknowledge the thoughtful responses from C.D., Toby and PeteinNippon. http://www.bmw2002faq.com/component/option,com_forum/Itemid,57/page,viewtopic/t,302054/

Since that post I discovered what I think is a weird quirk with this brake system. I decided to rebuild the rear wheel cylinders. I found that the rearward piston in each cylinder was extremely tight and probably not moving as they should. I turned the pistons to free them up, examined the bores, cleaned them with brake cleaner and re bled the system. The front brake drag problem completely disappeared! I drove the car for approximately two weeks without the issue. But I could not leave well enough alone.

Because I had not changed the wheel cylinders or brake shoes for a long time and because the handbrake cables are uneven and stretched, I felt guilty and ambitious enough to change the hardware on the rear brakes including shoes and new wheel cylinders. After renewing all of the above and re bleeding the system, the slight drag returned to all four wheels! It is very slight, but after a warm up drive, you can jack up each wheel and there is a “sight” drag at each wheel.

My neighbor volunteered that a proportioning valve (of some sort) may be stuck and has to be purged and bled. (He mentioned he just did this with one of his Fiats). I am not aware of a proportioning valve in line although I see a “distribution piece” or a glorified “T” fitting for the rear brakes here: http://www.realoem.com/bmw/showparts.do?model=ST16&mospid=47140&btnr=34_0968&hg=34&fg=15. Nor am I aware that there is anything moving in this part to go bad. I tried adjusting or checked the adjustment on the rear brakes, including the hand brake, the brake pedal linkage, and even the linkage as it is attached to the master cylinder.

I went back to the original responses and looked at the Haynes manual (pp. 127-130) and noticed something called a “pushrod shim.” Haynes says: “On tandem type master cylinders, a clearance must be maintained between the end of the push rod and the master cylinder of .002 in. or (.5 mm).” I felt for the gap by feeling “slack” from the rear side of the booster (where the linkage is attached) and determined by feel that the gap was probably at least 1 mm. The “clunk” was there and it caused me to even regrease the point where the push rod meets the master cylinder. Now I am wondering if I missed something.

First, how does one actually measure the gap described in the Haynes? (I can't locate my blue notebooks from storage.)

Second, are the shims available and in what sizes? (I searched the OEM and could not find any such part on the realoem http://www.realoem.com/bmw/partgrp.do?model=ST16&mospid=47140&hg=34&fg=25

Third, would substituting a much thicker “o”ring between the master cylinder and the booster achieve the same effect (a larger gap)?

As I mentioned, the problem disappeared when I played with the wheel cylinders and I did not make any changes to the master cylinder or the booster. Nevertheless, since I do not want to be forced to replace the new parts with old questionable ones, I am willing to consider the linkage gap approach. I am sorry for the length of this post, but I would rather have you knowledgeable people consider a more complete picture rather than guess about a more terse one.

Thank you in advance.

Date: 7-16-08 10:24
From: DanOKC

Subject: Re: Slight brake drag - the continuing saga

If you got it to disappear after redoing the rear cylinders, I wonder if you maybe
mis adjusted the rear eccentric bolts & have drag from that. I know it was mentioned before, but it's easy to think you have them right when you have one shoe pushed to far out. If you really have the master cylinder pushing pressure after everything is warm, I'd suspect the booster itself, just because it's 30 years old. As far as the shim & the clearance, another 0.5mm over spec shouldn't be causing the problem, I've never heard of anyone buying shims for that either. Also easy maintenance for the calipers if they are sticking, grab the pistons with pliers & rotate them side to side, see if they are 'free', if not, you may have problems there too. Good boosters are hard to find used, compared to all the other parts, I'd check the front calipers out, maybe it's combination of both & you got the rears fixed, but now need to fix the fronts. A little WD-40 can help if you have sticking pistons on the calipers, & just working them back & forth with channel lock pliers, try not to rip up the rubber boots. Also how about the pedal linkage itself, make sure it's not binding. Just a few thoughts.
_________________
2002 owner since 1980

Date: 7-17-08 12:07
From: RudyB
Subject: Re: Slight brake drag - the continuing saga

Thanks for the thoughts. Calipers and wheel cylinders are very pliable and not sticking in the slightest. I adjusted the rear drums to the point where the eccentrics are backed off as far as possible - so much so that the drums are incredibly loose. The result is a lower pedal.

As you noted, the shim is .5 mm and while that does not seem like much, I have to assume Haynes got it from somewhere. Timing has prevented me from going through storage for the Blue notebooks to confirm what Haynes says and to obtain more info. Respecting the booster, I know it holds pressure well and, to rule it out I actually drove around with the vacuum fitting disconnected and - the slight drag was still there when everything is warm (operating temperature).

About the only thing I havent done is rebleed the system or go back to the tired wheel cylinders. Although it seems counter intuitive, I do know that air in the system might result in dragging brakes, but the system has been bled clean without any bubbles and the pedal is rock hard.

Date: 7-17-08 12:31
From: stone02

Subject: Re: Slight brake drag - the continuing saga

Hi,
have you replaced all four (4) flexible
brake lines? if you have not then do this first.
next make sure that the brake pedal ACTUALLY
does return all the way. also check the return
spring in the foot pedal box behind the pedal
mounting point. if this spring is not tight then
reset it . also make sure that the pedal box
is empty behind the pedal armature and below
this area inside the box also. i have seen junk
slide back and forth under the brake lever.
next loosen the bolt that the two pedals are
secured with only one sixth of a turn. then
under the hood behind the booster is a bolt
that is part of the brake linkage. loosen this also 1/6
of a turn. both these bolts are affected by heat
and moisture.these are tricks that i have used for years
to find brake problems.
once you find the point of binding replace or clean
and reset all the parts that you have messed with.
PLEASE post a followup. thanks
good luck
stone

Date: 7-17-08 12:33
From: stone02

Subject: Re: Slight brake drag - the continuing saga

Date: 7-17-08 01:19
From: RudyB
Subject: Re: Slight brake drag - the continuing saga

Thanks for the additional thoughts and making me hungry for a cheesesteak.

All (six) of the flexible brake hoses have been replaced quite some time ago with stainless clad aftermarket hoses, so I doubt the one way check valve issue comes in to play here. I suppose they can go bad, but they do not show any outward signs of wear, nor is there any indication of potential ballooning or cracking that occasionally appears with the old OEM hoses.

As far as the linkage is concerned, the pedal housing is clean and about the only thing it has too much of is oil, which was used to ensure no binding. You will have to take my word for it, but the pedal return is strong enough to use as a catapult for small animals. The pedal does not bind and fully returns to its upright position. There is a slight amount of play in the linkage that I left along time ago to rule out any possibility of early pedal engagement. What I cannot say for sure is what play exists (if any) between the master cylinder and the booster, but I tried to check this by pushing and pulling on the post within the booster and it seemed as though there was a little lash. Because I greased things fairly well, I assume there is even more lash there that is being masked by the grease. As noted, the Haynes manual specifically mentions a "shim" but it neglects to describe the shim or explain how things function without it or with more or less clearance than specified (.5 mm).

Having addressed your concerns, let me remind you that I drove two weeks on the old wheel cylinders and shoes and there was no drag felt. There had been a slight drag prior to that, but it went away with the tinkering of the wheel cylinders. It may be that something else was adjusted that I have overlooked, but I can't think of it. I know that I did not touch the linkage, but to be complete, the tires were changed, the car was washed and waxed, and I replaced the drag link.

One more thing. The fronts are saddled with Repco/Axxis metal master pads (that supposedly improve when warm) but the rear friction material is good old generic ATE 9582's.

Exactly how or why the freeing up of the old wheel cylinders impacted the entire brake system (including the front calipers) remains to be explained. Why it (the very slight resistance/drag) continues after replacement of the wheel cylinders with brand new (very fluid pistons) is equally curious. The car brakes straight and quickly as well it should, but if you push the car from a standstill (when warm) it is as though someone is lightly riding the brakes. Makes me kind of glad most of the other cars in the stable are 4 wheel disk!

Thanks

Date: 7-17-08 04:03
From: RudyB
Subject: Update for Stone

For Stone

Per the manual:

"When replacing master cylinder: check clearance between thrust rod and master cylinder piston with plastigage and if necessary set to 0.5 mm with shim behind the plunger head."

In the trouble-shooting section:
"Brakes overheating severely when car is driven (brakes dragging?)

a) Equalizing passages in master cylinder blocked.
b) No clearance between thrust rod and master cylinder piston.
c) Incorrect brake fluid
d) Vent hole on brake fluid reservoir blocked."

It is interesting that there is no mention of bad/old brake hoses or poorly adjusted linkage or bad pedal return spring - all of which might cause of contribute to dragging brakes. Nor does the guide mention sticking calipers or wheel cylinders. I still can't believe it is the thrust rod - master cylinder clearance since playing with the wheel cylinders seemed to completely vitiate the problem. Nevertheless, I will be looking for my plastigage and separating the master cylinder from the booster probably this weekend.

Date: 7-17-08 05:17 From: Arizona002 Subject: did you grease the backing plate where the shoes ride on? (n
(nt)

Date: 7-17-08 07:35 From: RudyB Subject: Re: did you grease the backing plate where the shoes ride on? (n
Yes, oui, si. AND with the drums removed, I have had someone depress the brake pedal and watched the shoes expand and retract just fine. I even resized the "W" spring to add greater retract tension.

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