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Master cylinder or brake booster?

Tiny
WELD
  • MEMBER
  • 2006 AUDI A4
  • 2.0L
  • 4 CYL
  • TURBO
  • 4WD
  • AUTOMATIC
  • 212,000 MILES
I am trying to figure out whether I need to replace the master cylinder or brake booster or both. When I turn the car on, the brakes need a little time and effort to start working. Once I pump the brakes a few times they come to life, and they function normally. In the past, also, the brake fluid in the reservoir gets low and I have to watch to make sure it is at the correct level.

Of course, I don't want to drive this way and I want to fix it myself.


Replacing the master cylinder isn't a problem since it is right where I can get at it and an easy fix. The master cylinder is much more involved. So, would the problem I described be a symptom of a bad master cylinder or brake booster?
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Thursday, September 8th, 2022 AT 9:43 AM

9 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,709 POSTS
Hi,

When you first start the vehicle, do you mean the brake pedal is hard to push? Or does it go to the floor and then pump up?

If it is hard to press, that would likely indicate a vacuum leak to the brake booster. It is common for a plastic nipple that is molded into the connector.

Check for a broken nipple on the vacuum line leading to the brake booster. The brake booster vacuum line is located between the engine and the bulkhead on the left side of the engine compartment, connected to the front of the brake booster unit.

I attached two pics below to help locate and identify if this is the problem.

Let me know what you find or if I am incorrect about what is happening.

Take care,

Joe

See pics below.
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Thursday, September 8th, 2022 AT 8:35 PM
Tiny
WELD
  • MEMBER
  • 5 POSTS
Hi Joe,

Thanks for that. When I start the car and push the brake pedal it's a little soft but not completely loose where I can push it easily to the floor. It firms up to "normal" after I give it a few pumps.

I looked at the part you pointed out and took a picture of it. I gently moved the hoses and there's nothing broken on them and there's no leaking around the hoses.

Weldon

P.S.

I my last email I wrote: The master cylinder is much more involved.. It should read: The brake booster is much more involved.
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Friday, September 9th, 2022 AT 10:57 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,709 POSTS
Hi,

Does it go to the floor? You mentioned it was soft when first started and then firms up when you pump the brakes.

You should have power assist as soon as the engine starts. Is that happening?

Let me know.

Joe
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Friday, September 9th, 2022 AT 8:37 PM
Tiny
WELD
  • MEMBER
  • 5 POSTS
Hi Joe,

The brake pedal doesn't go all the way to the floor, but yes, there is power assist. It gets firmer the more I pump the brakes.

W
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Monday, September 12th, 2022 AT 9:31 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,709 POSTS
Hi,

Do me a favor. It's difficult to determine where the issue is because if it doesn't go to the floor, it sounds like the master cylinder is good. If you have power assist, that appears to be working. So, here is what I want you to try. Pump the brakes so you get a good brake pedal and then hold pressure on it. If there is a leak or issue with the Master Cylinder, the pedal may slowly drop to the floor.

Let me know if that happens.

Take care,

Joe
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Tuesday, September 13th, 2022 AT 12:39 PM
Tiny
WELD
  • MEMBER
  • 5 POSTS
Hi Joe,

ok. Started the car and the brake pedal has pressure. It doesn't go to the floor. When I hold pressure to the brake it maintains most of the pressure but recedes a little. It doesn't go to the floor.

W
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Sunday, September 18th, 2022 AT 10:31 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,709 POSTS
Hi,

If the brake master cylinder was the issue, I would have expected it to slowly drop to the floor. However, I still need to better understand something. When you say you need to wait for a bit and when pumping the brakes, they eventually come to life, what exactly happens that gives you that feeling? Do you start getting power assist?

Joe
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Monday, September 19th, 2022 AT 6:46 PM
Tiny
WELD
  • MEMBER
  • 5 POSTS
Hi Joe,

if I start the car when it is cold and put it in drive the car moves slowly forward. I push on the brake pedal and the brakes work but you can tell they are very weak. What I usually do then is put the car in park, pump the brakes and after about 6 pumps the brakes grab normally. What I usually do is just to let the car run for a bit while in park and then push on the brake pedal.

Are you thinking it might just be a faulty Brake Power Booster Unit Check Valve?
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Monday, September 19th, 2022 AT 9:04 PM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,709 POSTS
Hi,

If the brake pedal is stiff at first and then becomes easier to press, yes. It sounds like the booster. However, if it is easy to push the entire time and they don't work at first and there is no change in the brake pedal resistance, then I think it's something different.

Let me know.

Joe
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Tuesday, September 20th, 2022 AT 6:01 PM

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