1987 BMW 325E Brake Pressure Build Up

I am experiencing a problem where the brakes lock up after about four to five miles of travel. I was told that the problem could be resolved by replacing the master cylinder. I did, but the problem
continues. The only way to get going, is to release the pressure via the bleeding valves. This works sometimes for an eighth of a mile or sometimes up to ten miles. I removed all the relays from the ABS unit with no luck.
Is there someone who has experience this problem? How was it resolved?
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On 24 May 2007 07:49:04 -0700, snipped-for-privacy@yahoo.com wrote:

Brake vacuum servo is leaking................
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Please, tell us more. I've never heard of this, and I'm not sure what part(s) you are talking about, and how they play with the rest of the system.
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Jeff Strickland wrote:

It sounds plausible. Pressing the brake pedal opens a valve to let engine vacuum pull the brakes on harder, so a leak there could cause such a problem.
That said, all cases of brakes locking on that I've seen have been caused by a stiff linkage on cars that have the controls on the opposite side to the master cylinder (ie RHD in a car originally designed for LHD). But in that case the pedal would stay down.
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On Thu, 24 May 2007 19:04:34 +0100, snipped-for-privacy@nowhere.com wrote:

When active in the trade I had a couple of cars in with similar problem of the wheels (4) locking after a short drive. Releasing the pressure by the bleed nipple as stated did get the vehicle mobile again but the problem in the first was traced to a malfunction in the release valve of the brake servo not letting air into the chamber when the brakes were released. Swapped out the servo and no further trouble - DID NOT REPLACE THE MASTER CYLINDER.
Second vehicle with similar problems had a leaky master cyl and needed either a new one or seals. We stuck a new one on and all was well for about 2 weeks then another tow in for locked brakes so I swapped the servo and no problems with the brake till the guy sold the car some 4 years later.
I agree there may be other causes but from experience it would be one of the first places to look as a seized caliper would NOT lock all 4 wheels.
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Brake master cylinder would not cause this.
What are the EXACT symptoms you experienced? 'Brakes lock up' is a pretty vague description.
Going by your current symptoms, it's nothing more that a seized caliper.

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Nedavno Haydon napisa:

I had a similar problem once with my Audi. All four wheels would lock after about 20 minutes of travel. It seems that a problem was that my "mechanic" filled up brake reservoir with wrong type of brake fluid, that caused problem with rubber seals, and return spring was not able to return cylinder back. I changed master cylinder and problem went away.
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Pretty strange behavior allright. I see that the Bentley manual does list this condition under trouble shooting and suggests that it may be caused by the master cylinder although I don't personally understand how that would happen. If it was a seized caliper it would probably only happen to one wheel unless the car has been sitting unused for a long period of time. Do all the wheels get hot or just one?? It seems like you have ruled out the antiskid controller and the master cylinder so the only thing left is the vacuum assist. You might try removing and plugging the vacuum line the next time it happens to see if that fixes it. Just remember that you won't have the power assist with the vacuum disconnected. Good Luck and please let us know what the problem turns out to be.

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Jack wrote:

The master cylinder has a relief port in it - that relieves brake pressure when you let up on the brake pedal. It basically opens the brake circuit(s) to the reservoir. This same port is also how the system is kept full as pads wear and the caliper pistons move out.
If this port is plugged up, or the pedal isn't returning fully to the top position, it will cause the symptoms described.
I would check:
1. Is there anything restricting the movement of the pedal? ie - can it return fully up? (I'm thinking something like the pedal bracket got bent, or the brake light switch is binding.) It doesn't take much to cause this condition.
2. Has the entire system had a fluid flush? If not - it should. It's possible there is some crap floating around in the system and it's managed to find it's way back to the relief port.
Other than that - we probably need better diagnostics. When the wheels lock up - do ALL of them lock up? (Temperature of the wheel is usually a clue..) If it's all - it's doing this at a point common to all wheel circuits - which narrows it down to the master cylinder (again) or the ABS control hydraulic unit.
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I'm not convinced this could cause the brakes to lock on, though. Drag slightly, yes. I'd be looking at a power assistance fault.
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Dave Plowman snipped-for-privacy@davenoise.co.uk London SW
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wrote:

Thanks Dave..................
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This is commonly caused by a hose that is broken internally. The fluid is allowed past but the "break" in the hose acts like a check valve and doesn't allow the fluid to return and the caliper piston to retract. This is not the case here as it would happen the first time you applied the brakes. I also believe it is caused by a blockage in the system. I've never seen a vacuum assist do this but I can't give a definitive no on it either... As stated before, disconnect (and plug/cap the vacuum line to the brake assist unit and see if the problem goes away. Please let us know what fixes the problem.
On Fri, 25 May 2007 16:00:36 +0100, "Dave Plowman (News)"

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On May 24, 10:49 am, snipped-for-privacy@yahoo.com wrote:

5/25/2007 - Thanks for the many responses. I will try the suggestions on Monday (Momorial Day)
To clarify - The symtom is like it has the hand brake connected to all four wheels, and is applied to the maximum. All four wheels get very hot if I try to push ahead before a complete stop. Whenever it is totally locked, and I try to move, it just burns the clutch and the car just squats.
I will post the results of the suggestions I have already received.
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On May 25, 7:21 pm, snipped-for-privacy@yahoo.com wrote:

06/27/2007 - As I promised, even though it's been so long, I did change the vacuum booster and that did fix my problem... Thank you all.
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On Wed, 27 Jun 2007 19:14:01 -0700, snipped-for-privacy@yahoo.com wrote:

Well there you are. Glad to be of assistance and I believe that the Brake Booster was a suggestion of mine following several similar instances of this problem in the past.
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