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Old 05-15-2013, 03:01 AM   #1
adam325i
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M50 Brake booster problem

After swapping M50tub25 in my car I have the following problem.

Booster is stock 325i moved over. And the pedal does NOT bind,

As I drive around, As the engine warms up to operating temp roughly 5-10 minutes, the brake pedal will start to activate the brakes right at the top of the pedal travel, instead of roughly half way down. It gets worse to the point where the car starts applying the brakes and locks them on without me doing anything.

First time I let the car sit for a day or so and brakes were off and found no problems, Fixed some other little things and tried a test drive, roughly same time same problem occurs. While the brakes were on by itself I decided to take the vacuum line off the the booster, I heard a rush of air going in and the brakes were off.

Tested out the check valve and it functions 100% fine.

Anybody have to as any clue whats going on here? I'm so confused.

Thanks
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Old 05-15-2013, 06:46 AM   #2
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Hasn't the geometry of the pedal travel changed by moving it over? If it takes less travel to make the same movement your symptoms make sense no?

Fluid is recent and bled well? Any vacuum problems elsewhere?

Can you adjust the clevis at all to make up for the difference if there is any?
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Old 05-15-2013, 08:10 AM   #3
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did you cut and weld the clevis so that the push rod is still aligned straight into the booster? If it's pushing on an angle it may be binding or something
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Old 05-15-2013, 10:38 AM   #4
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When I moved the booster over I took out the brake pedal and drilled a hole right through used a sheer bolt to move the rod over. So there's no issue there. When I first drive the car around the pedal is normal.

it's like the booster has too much vacuum and won't release the pressure, hence when I took the vacuum line off the brakes released and I heard air going into the booster.

How dangerous or other concerns should I have if I remove the check valve?
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Old 05-15-2013, 12:59 PM   #5
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Removing the check valve will mean you won't have vacuum assist at part and full throttle. Sounds like something is binding, either the pedal pivot or even something in the master itself.
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Old 05-15-2013, 05:21 PM   #6
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The only thing I don't understand though is its fine normal pedal for the first 5-10 minutes with lots of braking. Then it's starts to come on and the brake engagement moves to the top of the hole. Is the brake booster supposed to have a calibrated leak? Since when I disconnect the vacuum line I hear air rush into the booster and it releases the brake.
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Old 05-17-2013, 12:17 AM   #7
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Anyone to the above question? I'm super stuck.
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Old 05-17-2013, 06:50 AM   #8
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The brakes apply themselves on their own and don't release? I would suspect the booster, somehow the vacuum is managing to move the pushrod on its own. There is a huge spring inside the booster that helps return the diaphragm to the rest position, do you remember if that got moved?
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Old 05-17-2013, 04:02 PM   #9
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Quote:
Originally Posted by Dissembler View Post
The brakes apply themselves on their own and don't release?
Correct. The pedal turns stiff. Brakes are applied automatically.


Quote:
Originally Posted by Dissembler View Post
I would suspect the booster, somehow the vacuum is managing to move the pushrod on its own. There is a huge spring inside the booster that helps return the diaphragm to the rest position, do you remember if that got moved?
I never separated the master cylinder from the booster itself. I only removed booster from firewall. The whole unit was never dropped while it was out, I had it wrapped up in a blanket.

http://www.roadkillcustoms.com/hot-r...oster-test.asp
Found this website, so I tried their tests. All test were conducted and had no issues.

Measured manifold pressure and came back at 19''

Its like there should be a vent or something that is clogged on the vacuum side, that should release vacuum pressure on the side of the booster?

I have no clue. Maybe is manual brake time? I'm lost.
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Old 05-18-2013, 06:58 PM   #10
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The booster doesn't vent to atmosphere but there are ports where atmosphere has to enter the booster, usually around the area where it bolts to the firewall. There will be some holes where air is drawn into the booster everything you step on the brakes.
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Old 05-21-2013, 12:59 PM   #11
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Ok, I replaced the booster and seems to work good now. The old one, the plunger that goes into the master cylinder wasn't in properly.

Thanks so much for your help.
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Old 05-22-2013, 09:35 PM   #12
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Awesome! Glad you got it fixed
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Old 05-22-2013, 11:01 PM   #13
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Also a dirty icv and bad 02 sensor were also in the mix. She runs great now!
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