Binding brakes/faulty master cylinder; mystery solved???

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Brucey
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Binding brakes/faulty master cylinder; mystery solved???

Post by Brucey »

Binding brakes/faulty master cylinder; mystery solved at last?

I've finally stripped out several 6er brake master cylinders, and definitely diagnosed the fault that has afflicted my brakes for several years now, which made the front brakes bind on anytime the car stood in traffic, seemingly only when the brake MC got hot.

I've stripped out three MCs and two servos in my quest for the truth here; and I couldn't have done it without the help of other 6er-philes. As well as all the comments of others (here and elsewhere) I am indebted to Jonathan Griffin for donation of a used MC and servo, and special thanks are due to Godwin Lawrence who very kindly donated a new and unused MC in support of the cause.

Confusingly there are at least two BMW part numbers for the 23.81mm left-side union Al bodied MC , plus there is a (v. expensive...) cast-iron bodied MC with an end/right side union layout which I believe is used only on post 9/86 build RHD models. There may be others. Also there are other MCs e.g. E32 type ones which I have not looked at, but all are believed to be similar internally, (albeit with a different bore size in some cases) so may be susceptible to the same fault.

For those who don't know, these MCs are 'tandem' ones. In normal operation the rear piston is pushed mechanically by the servo, but the front piston is pushed hydraulically by the brake fluid displaced by the rear piston. Should either circuit fail, some braking remains on the remaining circuit (albeit with a long and/or soft pedal) because both pistons have limited travel even when there is no fluid; the rear piston can bear against the front piston, and front piston can't go beyond the end of the bore, thus some pressure can develop in either circuit even if there is none in the other. The circuits are diagonally split (but not on E12 base models..) typically so that the rear piston works the front right and rear left brakes. To prevent total fluid loss from one failed circuit, the fluid reservoir is internally divided.

It is noted that comparable (but not identical) faults can arise through a bad servo instead, and that there are other ways in which the compensation ports can be blocked. However it is the author's belief that the vast majority of E24/E28 'dragging front brake (s) when hot' symptoms may be caused by this exact fault.

This post contains headlines of the fault condition, including fault diagnosis (see items 4,5 & 6), followed by a MC stripdown guide with pictures.

The headlines;

1. It was a bad MC as suspected.

2. There was almost nothing 'wrong' with the MC when I took it apart.

3. However when hot the MC pistons failed to return fully, leaving pressure in the brake system.

4. This made for a 'dead spot' at the top of the pedal travel, followed by a 'click' as the pedal firmed up part way through the stroke, as the servo pushrod contacted the rear piston again.

5. To diagnose, I quickly disconnected the MC from the servo when hot (and brakes binding), leaving the brake pipes connected. I could see the MC rear piston shaft had not returned to its normal position.

6. As I watched it cool down, the shaft returned to its normal position and the brakes freed off. Others have reported that they similarly sometimes hear a distinct 'click' as the system cools off and this accompanies resumption of 'normal' braking.

7. The fault appears to be that the phenolic spacer/shaft guide at the rear of the MC swells over time (probably with oil or brake fluid contamination) to the point that it starts to bind on the shaft, but only when hot.

8. On test, an ovalised shaft measuring no more than +0.03mm would not pass through the phenolic spacer from a used MC without a hammer to persuade it.

9. This fault is more likely with a cast-iron MC body than with the aluminium one because of the differential CTE (Coefficient of Thermal Expansion) effects. If we assume that the spacer is radially and axially constrained, and CTEs are 21ppm/deg C for the Al shaft, 19 for the cast Al body, 27 (average) for the phenolic spacer, and 10.8 for the cast iron, then for a +60C warm up, the shaft expands +0.021mm, and the phenolic spacer bore constricts 0.021mm for the aluminium body, but 0.044mm for the Iron body. Thus the net clearance reduces 0.042mm and 0.065mm for each master cylinder type. It is estimated that the original clearance is 0.15-0.2mm, so a 'swelling' over time of the phenolic material giving just 0.1-0.15mm bore constriction will cause trouble for an Iron bodied MC and a little more will also cause trouble for an Al bodied one.

10. If I had known, I could have removed the spacer, sandpapered the bore slightly, and fixed it.

11. If the shaft sticks more than ~4mm from the end of travel, both compensation ports will likely be shut and both front brakes will bind. Here the pressure in the brakes is governed by the degree of MC binding. At its worst the car may be undriveable when the MC is hot, OK when cold. This is the fault that I had.

12. If the shaft sticks in the last ~2mm of travel, it is possible that only one compensation port is shut, so only one front caliper will drag. Here the pressure in the brake circuit is limited, typically by the front spring pressure vs. the rear spring pressure. Here the car may be driveable but one front brake will get very hot indeed. If the rear piston compensation port is closed, the typical pipe layout would suggest that the right side front caliper will bind.

(NB. In neither instance above are the back brakes likely to be affected, because there are bias valves in the brake system that cause the rear brakes not to work hard until there is substantial pressure in the system).

13. Since there may be a step change in system pressure (and therefore restoring force on the rear piston) as soon as the front compensation port opens (allowing the front piston to 'float' between the springs a little) the fault condition which afflicts just one brake circuit is quite likely to occur if the front compensation port can open before the rear.

14. There are two distinctly different mc styles; one with two 'conventional' compensation ports drilled through the MC wall, the other with a conventional port on the rear piston, but a 'poppet valve' type on the front piston. It is not known if one is more likely to give the fault than the other, but it is surmised that the 'poppet' type may more likely give the 'one brake dragging' effect, most likely on the front right caliper. E12 chassis models have a different MC which can give a similar fault condition for different reasons; e.g. Drew has reported that there is a screw-adjustable piston spacing in some versions of this MC, and that the screw can come loose, back out, and make it impossible for the front compensation port to open.

15. Note that when the MC piston shaft is binding in the spacer, you cannot wobble the shaft from side to side at all. However in a new MC, the shaft will wobble from side to side a small amount. This can be used as a secondary test for shaft binding.

16. The only other fault I had with mine was that the 'fill port' on the rear piston had been 'nibbling' the rear piston seal, which had created a lot a black rubber debris in the brake fluid. This can be rectified as explained in section 29.

Stripdown guide;

17. First syphon excess fluid from the reservoir. Then, if you can, put a plastic bag under the reservoir cap to make it more or less airtight.

18. Loosen the brake pipe unions slightly.

19. Remove the bolts holding the brake MC to the servo. Note that any use of the brake pedal from now on is to be avoided since it may result in the servo spitting the pushrod out at speed along with some of the oil. Ask me how I know this...

20. Remove the pipe unions, plugging each hole if possible. If no plugs available, make sure that dripping brake fluid doesn't go on paintwork; it is an efficient paint stripper.

21. On the bench now, drain and remove the reservoir from the MC. It will be tight, so it is a good idea to use a little penetrating fluid on the joints first. A little wobbling of the joints will allow the fluid to penetrate after which the reservoir should come off fairly easily.

22. Work the MC piston a few times with the holes unplugged but covered with absorbent rags, to remove the brake fluid inside. If the MC has a 'poppet' front piston, compress the MC and remove the retaining pin from the front fluid port, having removed the grommet and the baffle beneath. The pin is difficult to grip, but is a sliding fit and generally comes out easily. If the MC has a conventional front compensation port there will usually be a front piston stop-bolt on the underside of the MC (instead of the pin in the port). Again, compress the MC and remove the stop-bolt. (NB. If the bolt is turned when the MC is not compressed, the bolt will mark the front piston, maybe tear the seals, and may even generate swarf inside).

23. Now for the tricky bits. Note that in the next steps you can convert your MC to useless scrap in a number of interesting ways, one of which is to deform or mark the rear piston shaft even slightly, so be careful. The rear of the MC will have either a snap ring that you can get hold of, or one you can't, because it is lugless. There may be a way of removing the latter easily, but if so I don't know it; I had to cut a small slot in the back of the MC to allow the ring to be popped out.

24. Do your best to make sure the phenolic spacer will come loose; use a little penetrant, and perhaps give the spacer a light tap with a hammer, using a suitable socket as a drift.

25. Now grip the shaft in a vice. To avoid crushing and marking, you must protect the outside and support the bore of the shaft internally. I used a suitable 1/4" drive socket internally, and used a couple of layers of 'scotchbrite' externally. Note that the supporting socket must be an exact fit, and that the bore of the shaft is not well-toleranced, so a socket that works in one MC may not work on another.

26. Now use a soft hammer to drive the MC body away from the rear piston/spacer assy. Be careful, and use more penetrant if required. Don't hit the MC mounting lugs! It is very easy to break a MC this way! Hopefully the spacer will come out OK, followed by both pistons. Note the order of the parts carefully. They will usually be like the ones in the first pictures but there is no guarantee. In particular note that the springs are usually different from one another. If there is a lot of corrosion present (likely) or wear (unlikely on the metal parts) think about getting a new MC or at least a rebuild seal kit. In the UK rebuild kits have been difficult to find for some time, but it is understood that they are presently available from 'BiggRed' who also offer exchange calipers and other brake parts.

27. It should be possible to check the spacer for clearance at this stage. If it is a little tight, it can be eased a little quite simply. Get a tube 1-2mm undersize, wrap it with 400grit abrasive, and clamp it horizontally in a vice with the spacer mounted on it. Now rotate the spacer by hand with a little downward pressure, so that you generate a little dust by abrasion. Stop after a small amount has been removed, and check the clearance. Repeat as necessary. You are aiming for ~0.15 to 0.2mm clearance. (I note also that a new spacer also has a 0.1-0.15mm clearance on the OD, where used ones are rather more snug; it may be a good idea to dress the OD as well.)

28. Check all the parts for wear, brake fluid deposits, corrosion etc; clean and refurbish as necessary.

29. Now is a good time to check the sharpness of the rear piston fill port. Typically this port is angled and has a very sharp edge which can damage the rear piston seal, so if your old seal was thus 'nibbled' you could address this. With a little careful work using a thin strip of 600 grit paper through the hole, the sharp edge can be relieved a tiny bit and this will make all the difference to the seal life and the rate at which rubber debris appears in the fluid. Obviously any burrs or grit in the bore need to be removed before you rebuild. I note that the newest MC I looked at (thanks Godwin!) has a relieved edge already, where the older ones did not.

30. Clean everything and reassemble, noting that with older MCs the longer spring generally goes in the front as per the pictures. Use brake fluid as a lube if possible. If re-using a lugless snap ring, consider reprofiling the ends to allow it to be removed more easily.

31. For speed, I cheated and didn't rebleed the system at the calipers; I just pressure-bled at the MC unions. I reckoned this was worth a try since all the fluid in the system was new, and I hadn't lost fluid from the pipes that I could see. This dodge worked OK for me!

Well that is it; I hope this is of use to others, and as ever, all comments are welcome, good or bad. I daresay it would be very useful for others to post here with details of local repair kit vendors.

cheers
Attachments
DSCF1824v3.jpg
DSCF1824v3.jpg (148.03 KiB) Viewed 40464 times
rear piston detail.jpg
rear piston detail.jpg (119.15 KiB) Viewed 40464 times
poppet.jpg
poppet.jpg (78.22 KiB) Viewed 40464 times
MC parts.jpg
MC parts.jpg (237.68 KiB) Viewed 40464 times
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Thunderjet

Post by Thunderjet »

Fantastic find Brucey. Great information. =D>
Area 52

Post by Area 52 »

Great write up Brucey...you need to author the next Bentley manual!
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Post by Pod »

Very interesting, Brucey. Thanks :D
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Godwin Lawrence
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Binding brakes

Post by Godwin Lawrence »

Excellent write up as usual Brucey.
:D
Godwin Lawrence
lsintampa

Post by lsintampa »

My brakes were binding when the MC got hot. Caused the front end to shake like heck whenever the brakes were sticking.

Replaced the MC and they don't stick, but I still have the shaking when I brake at 40-50 MPH.

Odd but glad the sticking issue is resolved.
mauser

Post by mauser »

Nicely written and well illustrated. Those great illustrations are especially good for us that are still struggling to understand all that technical terminology on english.
carigbmw635

Post by carigbmw635 »

Hi All, after years of no problems, I am getting this problem with my 635. I did the brake lines and replaced both front calipers before finding this thread and need to do the master. My question comes down to the different manufacturers, pricing and performance. I have seen everything from Cardone, ATE, Centric, Beck/Arnley, Raybestos, etc. Prices range from $65 to $300, $54 for a rebuild kit. Can anyone explain what I am going to get for the price difference or do I just go cheap (cardone $65).
I do have ABS.
Thanks,
Craig
Area 52

Post by Area 52 »

My non-abs master in the ETA is the original ATE (hmmm...realized ATE is ETA backwards). In October it will be 28 years old. A new ATE master at AutohausAZ is $231.55.
Chris Wright

Post by Chris Wright »

I believe you can use the e32 MC also, it's around $172 at AutoHaus. Someone will have to chime in with the correct year/model or part number?

The upgrade to the E32 master cylinder, which has a larger 25mm bore size, is used to prevent pedal drop in big brake upgrades. In a standard system the larger bore E32 MC will cause a smaller pedal travel then is standard, but several have used it with no reported ill effects.
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clipper47
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Post by clipper47 »

What a great writeup. The driver side front brake seems to drag so maybe a rebuild or new M/C is in order for me also. The E32 M/C might be a good upgrade if part number is available.
Dave
_____________
1985 635CSi US
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Area 52

Post by Area 52 »

clipper47 wrote:What a great writeup. The driver side front brake seems to drag so maybe a rebuild or new M/C is in order for me also. The E32 M/C might be a good upgrade if part number is available.
The caliper piston could be binding. When the pedal is depressed, the piston extends out toward the inside pad as it should, but it doesn't retract back into the caliper cylinder due to crud and rust on the piston where it rides across the square o-ring just inside the outer lip of the cylinder. I would pull the wheel off, remove the caliper and inspect it before investing in a master that you may not need.
Also, I can't stress enough the importance of bleeding the system every two years...your calipers and master will last much longer. Also, I strongly recommend a pressure bleeder.
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clipper47
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Post by clipper47 »

Area52, Thanks, I will do that although I had the caliper off the rotor a few months ago and didn't notice any crud or rust did. I also did bleed the brakes using my pressure bleeder as I do annually. I guess a test would be to check whether it is dragging when hot after a trip and cold after car has been sitting for a few hours?
Dave
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nsxguy7
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Dragging brakes

Post by nsxguy7 »

Wow...many thanks Brucey as the day before you made this post, my 6er developed these exact symptoms. I decided to upgrade to the e32 MC since I planned on upgrading to bigger brakes in a few months.
My question is this: when I unbolted the MC from the servo, I noticed a small amount (maybe half a teaspoon) of clean power steering fluid just inside the servo opening. I wiped it away but about an hour later, a like amount had accumulated in the same place. Do I have a leak in the servo, or is this normal? Thanks in advance for the help.
Area 52

Post by Area 52 »

You have a leak either on the large black cup o-ring or through the seal in the end of the large black cup of the brake booster. More than likely it is the large o-ring.

I bought a couple of kits from Jay Stratton and installed them on my 735 and another 535. I decided to just replace everything since I didn't want to have a failure of another seal soon after replacing the large o-ring on the cup. The rebuild is soooo simple...just follow Jay's pictorial instructions. Jay also will rebuild it for you if you prefer for an outstanding price. Here's Jay's writeup with links to pictorial:

viewtopic.php?t=17387&highlight=
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JCS
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Post by JCS »

NSX guy,

Area 52 is very correct about the possibilty of the caliper pistons sticking, mine were really stuck, big time.
There also was a recall out on my M6, the master cylinder failing to release.
And I also upgraded to the 750i M/C.
And if your Booster is leaking ATF, then you made the right call.

Thanks for the PM.

I just returned your PM.

Here is the link to the RECALL.
http://www.allworldauto.com/inv/NHTSA_1 ... 73163.html
Jay
'88 M6, Red/Tan
My NEW Website https://www.jaysbmwparts.com/
deacs

Post by deacs »

Brucey when you say you had to cut a small slot in the master cylinder to remove the snap ring what exactly do you mean? I've remkved my master cylinder and have the same snap ring with no 'ears' as such. I can't for the life of me figure out how to get the snap ring out
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sansouci
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Post by sansouci »

Is this problem more characteristic of RHD vehicles?
Sansouci
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Brucey
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Post by Brucey »

if you cut slot in the end face of the MC you can access the snap ring from the other side and this should help you get it out. Drilling a small hole radially above the snap ring will also allow you to remove the snap ring more easily. However that is only half the battle; getting the guide bushing out can be incredibly difficult.

Re the LHD/RHD thing; I think it is worst on post 9/86 RHD vehicles simply because the MC sits above the exhaust manifold and heat is part of the problem. But plenty of LHD vehicles have had the exact same thing happen too.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
deacs

Post by deacs »

Brucy how does one change the seals on the piston part with the poppet valve? I've got it stripped down and have a seal replacment kit but need to get the 3 seals on the piston part off. Does the piston split down somehow?
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Brucey
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Post by Brucey »

I could not see how to strip the piston down any further IIRC. I suspect that you just have to be brutal and stretch the new seals over.

BTW do you think the MC bore is in good enough condition? I have had MCs that leaked (pedal goes to the floor) and when I stripped them down the seals appeared perfect even under a microscope; the conclusion was that the bore was corroded.

cheers
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deacs

Post by deacs »

Not even a hint of corrosion on the master cylinder or piston to be honest.
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Brucey
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Post by Brucey »

the piston itself will nearly always look fine but then it only has to seal in one or two places where the rubber seals bear against it. If the piston is fine and there is no damage to the rubber seal lips under a microscope, you must infer that the bore of the MC is bad.

It is very difficult to see the kind of corrosion in the bore that will cause MC to leak down; you stand some chance with a binocular microscope and an angled mirror on a stick.

cheers
~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ron
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Post by ron »

JCS wrote: .....and I also upgraded to the 750i M/C.
These stick as well. Just had to replace mine. Brucey has my old one and is going to strip it to check why.
They are ALWAYS rustier than you thought!!!!!!
'85 M #228
'87 M #367
'88 High line.
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deacs

Post by deacs »

Regardless it's worth a try ah the rebuild considering the absolutely rediculous cost of a master cylinder for it...... Unless uou know where I can get one new for sensible money. By sensible I mean less than £100!!!
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