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Gasket in Master cylinder cap keeps popping out

Tiny
J0LKEEPOLDCAR
  • MEMBER
  • 2012 VOLKSWAGEN ROUTAN
  • 3.6L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 150,000 MILES
Dear Sir:

Recently I noticed the gasket inside the master cylinder cap keeps to pop out even after I push it back in every time. Brake works normal to me and I did not notice any change from before. BTW, I bleed brake fluid and changed read brake pads about a year ago.

I did not find any leaks from bleeder screw. What could be the cause why the gasket does not stay inside the cap? There must be some leaks, although I did not touch any part of the brake system except brake pad change and brake fluid bleeding. I also unplug brake booster hose a few times for other problem.

Best regards,
Jason
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Monday, October 12th, 2020 AT 7:32 PM

8 Replies

Tiny
KASEKENNY
  • MECHANIC
  • 18,907 POSTS
Can you get a picture of this gasket? Is it the rubber seal around the cap shown below?

If so, I suspect this cap has been contaminated and the rubber has expanded. From what I can pick up, you are suspecting that something is pushing the seal out? Like there is excess pressure causing to pop out?

Let's get a picture of it and go from there. Thanks
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Monday, October 12th, 2020 AT 7:46 PM
Tiny
J0LKEEPOLDCAR
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Thanks a lot. Here are some pictures.

First two pictures are how it looks like when I took it out.

Last picture is I pushed it back.
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Monday, October 12th, 2020 AT 8:07 PM
Tiny
KASEKENNY
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Got it. I suspect the rubber has swelled as it is covered in brake fluid. Clearly this is meant to make contact with brake fluid but I have a Chrysler product and this is what my cap looks like. FYI - your van is a Chrysler Town and Country with VW badges on it so the cap is basically the same.

I would replace the cap just to be safe because when I opened mine it looked just like shown in the pictures.
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Tuesday, October 13th, 2020 AT 6:13 PM
Tiny
J0LKEEPOLDCAR
  • MEMBER
  • 5 POSTS
Hi KASEKENNY1, really appreciate your help. My car is indeed a Town and Country or Caravan.

I am getting a new cap and hopefully the fix would be this simple.

After you talked about "contaminated", I did some google since it seems pretty serious.

Hopefully I don't have contaminated brake fluid by oil. Some pic and post on the internet about those are scary.

Will try to mix some of my brake fluid with water to see whether it will separate and float to the top. If this happens, my brake would be in a very bad situation.

Cross fingers and wish the best.
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Tuesday, October 13th, 2020 AT 7:09 PM
Tiny
KASEKENNY
  • MECHANIC
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Well, I mentioned that because that is a common occurrence but I didn't hear anything that happened that would make me think that it was actually contaminated. Is there a chance that you added oil or something else by accident to the brake fluid.

Did you add brake fluid to the reservoir? If so, what did you use? This is where VW and Chrysler differ. VW calls for a DOT 4 fluid and Chrysler calls for DOT 3. Chrysler does say that if DOT 3 is not available you can use DOT 4 so they are compatible but it just raises the question of what fluid you used. Thanks
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Wednesday, October 14th, 2020 AT 7:47 AM
Tiny
J0LKEEPOLDCAR
  • MEMBER
  • 5 POSTS
I just used DOT3 as owner's manual. I don't think any other fluid was added, though the car was maintained at VW dealer during its warranty period.

Some post on internet claims this could be due to leak or piston movement, the gasket or bellow could expand to compensate the pressure to prevent moisture enter master cylinder.

Will mix top fluid from reservoir with water to see whether it has oil inside.

Will update soon.
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Wednesday, October 14th, 2020 AT 7:54 PM
Tiny
J0LKEEPOLDCAR
  • MEMBER
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Got some brake fluid out and mixed with water and they didn't separate to 2 layers. I guess no oil mixed inside. Though the color was very bad.

Used brake stripes to check the fluid and it seems acceptable.

I also noticed the gasket pop out even though didn't drive at all since I push it back last time.
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Thursday, October 15th, 2020 AT 1:08 PM
Tiny
KASEKENNY
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So I have seen the gasket extended on different vehicles and it normally is due to the brake fluid being low from the brake pads being worn out. However, yours was covered in brake fluid and it looked soft which is why I made the comments I did. Plus the fact that it would not stay in place.

However, I think this may be a moot point if the fluid needs changed. I would get it flushed and put DOT 3 or 4 and then put the new cap on it and see what happens.
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Thursday, October 15th, 2020 AT 8:06 PM

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