Yet another brake problem.

[Jason Bates]

I have an 86 635csi with a manual transmission. After I got the car i started to fix a few things here and there. I pulled the brake booster to put new o-rings and a seal in it to fix that leak. Put it all back together, my master cylinder fails. Put a junk yard m/c in from a 750il, works great for months. Starts to warm up where I live, still working fine. ON a warmish day, I take lunch from work and my brake pedal is rock hard, and brakes are locked up stiff. This has been happening off and on, randomly for a while now. The pedal even starts to get hard and brakes lock up when driving, again randomly. I can drive for hours and they would be fine. Drive in to town, they lock up on me as soon as I park. I put a new master cylinder in next day, locks up.

Long story short, I have no clue what is causing this. Any ideas on what to look at or replace now?

[pthm]

air in the system,

[carnut1947]

On an American car, the cause would be not enough clearance between the push rod and the master cylinder keeping the master cyl. piston slightly depressed. Not knowing whats between the master and the hydraulic booster on a 635, is it adjustable? Or is the rod to the brake pedal adjustable? Doesn't sound like air in the system to me. I've also seen a misadjusted brake light switch cause this.

[Jason Bates]

air in the system,

Air in the system would cause a soft pedal. pumping it up would compress the air in the system. I have the opposite problem.


On an American car, the cause would be not enough clearance between the push rod and the master cylinder keeping the master cyl. piston slightly depressed. Not knowing whats between the master and the hydraulic booster on a 635, is it adjustable? Or is the rod to the brake pedal adjustable? Doesn't sound like air in the system to me. I've also seen a misadjusted brake light switch misadjusted cause this.

From what I've seen it is not adjustable, just a solid rod.
How would a misadjusted brake light switch cause something like this? Do you mean the pedal adjustment its self, or the actually light switch?

[carnut1947]

If the brake switch is too far in against the pedal, it would push ever so slightly on the brake pedal which could cause the master cylinder rod to not return to the by pass position when it was released. Could there be an O ring or shim between the hydraulic booster and master that wasn't switched over when the master was replaced? I've never had my brakes apart. Just bought my car. These are general brake hydraulics educated guesses. Have you confirmed the 750 master cylinder has the same part number?

[Jason Bates]

If the brake switch is too far in against the pedal, it would push ever so slightly on the brake pedal which could cause the master cylinder rod to not return to the by pass position when it was released. Could there be an O ring or shim between the hydraulic booster and master that wasn't switched over when the master was replaced? I've never had my brakes apart. Just bought my car. These are general brake hydraulics educated guesses. Have you confirmed the 750 master cylinder has the same part number?

I don't think the light switch is the problem. I've had this car almost a year now and its only just started doing this in the last four months or so. There isn't any o-rings or shims between the two. The 750 m/c is actually a larger diameter than the stock one. Many people have swapped them over as an upgrade.

[OBwan]

I always thought a hard pedal in these cars is the brake bomb.

[DesktopDave]

I think you have a dead bomb (pressure accumulator) too. When the accumulator bladder loses pressure, it won't 'store' any boost. The pedal gets rock hard since there is no power assist. Since the P/S pump is a dual-circuit unit, the power steering will still function.

I don't believe they are rebuild-able in the conventional manner, I suspect a local hydraulics shop wouldn't be able to test it out. I have a complete spare system I'd offer you but I'm not sure if it works.

[Jason Bates]

I think you have a dead bomb (pressure accumulator) too. When the accumulator bladder loses pressure, it won't 'store' any boost. The pedal gets rock hard since there is no power assist. Since the P/S pump is a dual-circuit unit, the power steering will still function.

I don't believe they are rebuild-able in the conventional manner, I suspect a local hydraulics shop wouldn't be able to test it out. I have a complete spare system I'd offer you but I'm not sure if it works.

I've been researching a lot more today cause I'm so tired of it. And that's what I'm starting to lean towards as well. But I still don't know. The very first time this happen, I drove to work, perfectly fine, came back out to lunch and brakes were locked up.

I'm starting to toss around the idea of a manual brake setup.

[DesktopDave]

Wait just a sec...if the brakes a locked up something has to be binding. A bad accumulator would cause a hard pedal but not locked up calipers. Are you sure the o-rings in the booster are still good? Any ATF leaks around the master cyl or booster?

I also recall Brucey over at BigCoupe determining that the E24 secondary piston in the master cylinder can bind and cause partial brake lock-ups, seemingly at random. This was for the stock E24 master...not a 750iL unit, but they're pretty similar. I'll see if that thread isn't still up, he's famous for excellent write-ups about the great solutions he comes up with. Here it is (http://bigcoupe.com/phpBB2/viewtopic.php?t=16351), great reading.

[Jason Bates]

I actually came across that one today. I pulled the booster and put new o-rings and a new seal in it, it leaks a little, I don't think I got the seal to seat in the right spot.
The brakes have started to drag slightly then enough to full blown lock up and keep me from moving the car at all. But this first started after the car being parked, after driving normally, then full lock when I try to move it after being parked for a few hours.

[carnut1947]

If the bomb is faulty it would affect boost, it would not lock up the brakes.Does the push rod attach to the master or to the booster? Do you still have the old master cyl.? If so measure the piston depth from the back of the master cylinders and compare them. If its attached to the booster then does it reach into the master cyl. at the same depth?

[Jason Bates]

If the bomb is faulty it would affect boost, it would not lock up the brakes.Does the push rod attach to the master or to the booster? Do you still have the old master cyl.? If so measure the piston depth from the back of the master cylinders and compare them. If its attached to the booster then does it reach into the master cyl. at the same depth?

Well, it's actually not attached to either side. Rather pushed on by the piston in the booster, then pushes on the piston in the m/c. I do have the old m/c's some where I could compare the depths.

I just want to get to the bottom of this. I don't want to keep throwing money at the problem.

[mauryc]

Does your pushrod have the tapered end? I know on my 88' you can put it in backwards. Don't know if this will affect the travel distance of the piston or not.

When the brakes are hot and locked up, crack a line at the MC and let some fluid out. See if this helps. Do front and back seperately.
Maury

[dmac156]

It sounds like something is wrong with that master cylinder. If the all the brakes are locking up then that rules out calipers, as stated earlier, a bad booster would make the pedal hard but the brakes would not apply. It sounds like a spring is missing, or something is in backwards (a backwards cup seal could do this) in the master cylinder. You can pick up an e24 master cylinder pretty cheap, I would ditch the 750, go back to stock and see if your problem is solved.

[carnut1947]

I agree! Get the proper master cylinder.

[DesktopDave]

I actually came across that one today. I pulled the booster and put new o-rings and a new seal in it, it leaks a little, I don't think I got the seal to seat in the right spot.
The brakes have started to drag slightly then enough to full blown lock up and keep me from moving the car at all. But this first started after the car being parked, after driving normally, then full lock when I try to move it after being parked for a few hours.

You're on the right track here IMHO. When a repair results in any side effects I always start by double-checking my own work. I'm not suggesting any lack of skill on your part, this is just a personal habit that's helped me a great deal. :confused I'm pretty good, but certainly not the most competent mechanic I know. That 750 MC is probably not the culprit. I've rebuilt a bunch of auto & motorcycle master cylinders and they're pretty hard to get wrong. From the little I've read about those Ate H31 boosters (link (http://www.bimmerforums.com/forum/showthread.php?1709393-H31-quot-Hydro-Boost-quot-power-brakes-steering-system)) (link (http://www.bmwccae31.com/DOCS/Brake%20Power%20Assist%20System.pdf)), they are very particular about seals and clearances.

If I had a dime for every time I had done something wrong, I'd be able to afford more parts! :shifty

[master6]

Jason.

So you got my repair kit for your booster. and installed it.
My experience with the brakes locking on started 5 years ago in my '87 m6, and also this summer with my newly acquired '88 m6.

The problem is your M/C! see this recall notice link http://www.allworldauto.com/inv/NHTSA_1987_BMW_M6_investigation_73163.html

And your booster should not be leaking, with my kit, the 38 mm o-ring can be cut as it slides past the Grub screw hole, did you take the steps I advise in my tutorial to prevent this?
Did you also cement the shaft seal into the black plastic cup, with the Rhino Glue as I suggest?

This summer I bought 3 M/C's
1. I bought a cheap Chinese M/C, as my brakes on all 4 wheel locked up, and the new one (750) failed within 48 hours as soon as it heated up, It was a URO from Rock auto ($72.) - FAILED.
2. I then bought (750) a Centric from Rock Auto ($133.) failed in 24 hours, also from China - FAILED
3. I then got an (750) ATE from Autohaus AZ ($181.) GREAT, problem solved!
Do not buy an M/C built in China, or the junk yard, only buy German!

Best,

[Sansouci]

+! to Jay's advice. Brakes are safety systems. No fooling around. There is another post (could be here or BigCoupe) where a Dad finished a rebuild and gave his son the car who totalled it due to brake failure (he tail ended someone else at a stop light)<br>--Ken

[Coupenut]

I'm curious, what is the benefit of the 750il (E32?) master cylinder over the stock one?

[Jason Bates]

So the m/c I put in over the weekend is an ATE I got from the guys at Guten Parts.

I don't think I got the shaft seal to seat in the right spot as you mentioned. I don't know if it's causing my problem, but I don"t think it's helping.

The only reason I did the 750 m/c is because it was substantially less expensive than the standard one.

I'm not sure if it's front brakes, rear or all four corners.

[master6]

I'm curious, what is the benefit of the 750il (E32?) master cylinder over the stock one?

Costs less

[Jason Bates]

So I had some time today. I found that it is all four wheels locking up. I haven't driven this car in three days, and yet again, the brakes were locked up to the point I had no way to move the car. Broke loose the forward most line on the m/c and could hear the brakes loosen up, same with the back line.

Is this the typical symptoms of a failed m/c?

[DesktopDave]

Still could be either the M/C or the booster, but you've definitely narrowed it down to those two. I'd be tempted to loosen up a hydro line on the booster next time, but I'm not sure which fitting would be safe to open. That ATF can be under pretty high pressure even when the car is shut off due to the accumulator.

[dmac156]

I doubt it has to do with the accumulator, that should always be providing boost pressure to the MC, but it will not move the fluid to compress the calipers, that is the job of the MC itself. The fact that you could loosen the line proves that the MC is holding the pressure on the brake lines. Either something is wrong inside your MC or something is mechanicallying pushing on MC, for example the brake pedal linkages (which would be unlikely).

[carnut1947]

I would have disconnected the brake pedal from the rod. I think its the master cyl. not fully returning to rest.

[Jason Bates]

So, should I try to find a rebuild kit and rebuild one of the 750 m/c's that I have now, or should I just get another 750 ATE m/c and hope it does the job for me?

[carnut1947]

I presume you've confirmed the master as the culprit? If so, get another master cylinder. I NEVER! rebuild brake parts. And get the proper master. the one designed for your car.

[Jason Bates]

I haven't really ruled out anything just yet. I really just need to get going on this seeing as it's been down for over a week.

I need to figure out what it is.

Does anyone have some trouble shooting steps that could help move me along?

[Bert Poliakoff]

Years ago,many, I restored an XKE. First time driving it the brakes totally locked up. The booster had a bleed valve on it and as I slowly started letting air/vacuum out of it, the brakes released. I then set it with a vacuum gauges to factory specs in the manual and no more problem. So, I am wondering if you did not put your booster together properly? I believe the article by Brucey on big coupe had to do with lack of breaking? He tore his m/c down and found the secondary piston was made out of a fibrous synthetic material that was falling apart. If I remember incorrectly, then the scenario was reversed and the piston swelled up and wasn’t releasing pressure. So maybe it is an m/c problem after all.

[master6]

Jason,

I can't understand, you are still unconvinced on this.
Did you not see the BMW recall notice link I gave you?

And thanks for telling everyone that you only used my quick fix kit, and you did not replace the o-ring at the fire wall end of your booster.
As that is were the leak is!

Best

[Jason Bates]

I did see the recall notice, and I put in an ATE master cylinder, as recommended.

And I used the full rebuild kit from you. It's not a light on you or your kit. I think I failed to install it correctly.

That said, I think I should get another kit from you and start over from there. As well as replace the master cylinder again. I'm Just trying to make sure and double check everything so I can get this fixed once and for all.

[carnut1947]

Have you called a BMW dealer regarding this recall? They just need your VIN number. Recall repairs are free.

[Sansouci]

Jay,
Could you post a link to the recall?



Jason,
Your posts keep referencing a m/c for a 750. Go with an ATE for an E24.

Good luck.

[1986series6]

I suggest you let Jay rebuild it for you.

He did mine, it works great and for what he charges

it's a slam dunk.

As far as the recall? After all these years, I doubt it!

But I may be wrong.

[wfwright2]

Do not want to send this thread off to a tail spin, but as far as using a MC from a 750. Besides the cost, some us who up-sized our brakes and/or want a shorted pedal throw, the larger bore in the 750 MC is a good choice, realizing that the brakes will become a bit more sensitive also know as twitchy to some. Just another way for us to keep our cars on the road and pleasurable for us to drive.

[master6]

[QUOTE=Sansouci;28861885]Jay,
Could you post a link to the recall?


Ken,

Here you go,
http://www.allworldauto.com/inv/NHTSA_1987_BMW_M6_investigation_73163.html


Best,