Brake Fluid Overflowing Reservoir

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Thread: Brake Fluid Overflowing Reservoir

  1. #1
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    Brake Fluid Overflowing Reservoir

    I just noticed that brake fluid fluid has spilled out of the reservoir, leaking from the cap eventhough it is tightly closed. On unscrewing the yellow cap the fluid is up to the brim of the reservoir. It'a 2003 Passat Variant with 1.8T. Did rear brakes at about 35K, has about 45K now. Fluid was flushed before that. No problems with the pad change or with the brakes that I've noticed. Any ideas??

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  3. #2
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    First of all, clean that fluid off of painted surface area really well. Brake fluid will eat your paint up.

  4. #3
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    Sounds like when your car went in for service the brake fluid was changed then topped off to the fill mark. Then when the brakes were serviced the caliper pistons were compressed in and pushed the fluid back into the brake system. IIR the reservoir is not air tight. Just get a turkey baster or syringe and remove the excess fluid. Clean the engine compartment of the fluid. No telling how long it has been like this but it could cause damage to paint ect.

  5. #4
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    I've found that I need to keep it at the "min" mark and not the "max" mark to keep it from spilling out during spirited or auto-x driving. If you look close you'll see a pin hole in the top of the yellow cover where it leaks from.

  6. #5
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    Quote Originally Posted by PGevanthor
    I just noticed that brake fluid fluid has spilled out of the reservoir, leaking from the cap eventhough it is tightly closed. On unscrewing the yellow cap the fluid is up to the brim of the reservoir. It'a 2003 Passat Variant with 1.8T. Did rear brakes at about 35K, has about 45K now. Fluid was flushed before that. No problems with the pad change or with the brakes that I've noticed. Any ideas??
    Wanted to pass on an update.

    I adjusted the fluid level to the bottom of the filler neck again and the next day after a run it was up to the top. Brought it in to what I think is one of the best dealers in the San Diego area and they checked it out and found nothing wrong. "If air were going in via a leak the pedal would be soft but it isn't." The level was adjusted and I took it home yesterday. Today it is overflowing again, leaking onto the top of the reservoir. I will bring it back but I need some ideas and apparently the dealer does also.

    I can't say exactly what they checked. They've never seen anything like it.

    Any brake experts out there? Thanks. Peter

  7. #6
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    Sounds like the reservoir is being pressurized. Bad valve action in the master cylinder?

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    Anyone know the cause of this brake fluid overflow problem? My 02 1.8T AWM has started doing this. Front pads are still original, rears were replaced 6 months ago including a full flush with Valvoline Dot 3/4 syn fluid. Pedal is firm.

    Reservoir level is halfway between min & max, but the neck with the fill cap is full to the top. Why wouldn't the fluid in the neck drain back down into the reservoir?

    I'll be replacing all the pcv valves, fittings & valve cover gaskets this weekend (leaking oil). That would rule out a bad check valve if the brake fluid still leaks afterward.

    Hope it's not the master cylinder or ABS...

  9. #8
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    Follow up with fix and long term results. Caused by two problems:
    1. Bad seal on reservoir cap let moisture and contaminants in causing cloudy fluid. New cap fixed that but didn't stop the overflow.
    2. Master cylinder failed internally causing pressure buildup that forced fluid out of the reservoir. Replaced master cylinder and no leaks for 2-1/2 years.

  10. #9
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    Overflowing can NOT be caused by the master cylinder.

    Brake fluid reservoir overflow is caused by pushing the pistons back when replacing brake pads.
    This should be avoided by sucking some fluid out, and top up to correct level at end of brake work.

  11. #10
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    Quote Originally Posted by Dan_N View Post
    Follow up with fix and long term results. Caused by two problems:
    1. Bad seal on reservoir cap let moisture and contaminants in causing cloudy fluid. New cap fixed that but didn't stop the overflow.
    2. Master cylinder failed internally causing pressure buildup that forced fluid out of the reservoir. Replaced master cylinder and no leaks for 2-1/2 years.
    I agree with Tomvw's post above and will add that in your case, ALL of the additional volume increase was due to the brake fluid being hygroscopic - a physical characteristic that causes the fluid to absorb moisture. When moisture is absorbed, there is a braking performance "hit" (lower fluid boiling point due to the extra moisture-lower fluid boiling points cause earlier brake fade and eventual pedal sponginess if severe enough), but most people completely forget about the volume increase. As an aside, this (little or no volume increase seen in the reservoir) is the main reason I chose to wait ~10-12 years to change the brake fluid on my '99 Passat.

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    Quote Originally Posted by Dan_N View Post
    Reservoir level is halfway between min & max, but the neck with the fill cap is full to the top. Why wouldn't the fluid in the neck drain back down into the reservoir?
    This was due to a reservoir breather blockage.
    Possibly caused by overfilling, the fluid could block the strainer holes and the gap around the strainer, air pressure would prevent the flow into the reservoir.

  13. #12
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    Quote Originally Posted by Tomvw View Post
    Overflowing can NOT be caused by the master cylinder.

    Brake fluid reservoir overflow is caused by pushing the pistons back when replacing brake pads.
    This should be avoided by sucking some fluid out, and top up to correct level at end of brake work.
    And to add what you've pointed out in the past, open the caliper bleeder when retracting the piston so old fluid doesn't flow back up the circuit.
    Tomvw likes this.

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    Quote Originally Posted by Tomvw View Post
    Overflowing can NOT be caused by the master cylinder.

    Brake fluid reservoir overflow is caused by pushing the pistons back when replacing brake pads.
    This should be avoided by sucking some fluid out, and top up to correct level at end of brake work.
    Except in the one case that does exactly this. A internal seal failure between the two independent hydraulic circuits will cause a loss of braking power to 2 wheels and pressure build up in the reservoir. ALL other diagnostics for testing a master cylinder will pass so almost nobody ever recognizes this failure case.
    cchief22 likes this.

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    we all need to learn to accept the fact that truth is stranger than fiction. strange crap happens, and that's why this board is such a great place to be some times

  16. #15
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    Quote Originally Posted by Dan_N View Post
    A internal seal failure between the two independent hydraulic circuits will cause a loss of braking power to 2 wheels and pressure build up in the reservoir.
    I didn't follow that, so could you explain further? The reservoir is vented to the atmosphere, so how would the pressure would build up?

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    Quote Originally Posted by ylwagon View Post
    I didn't follow that, so could you explain further? The reservoir is vented to the atmosphere, so how would the pressure would build up?
    Where's the vent and how do you plug it when using a Motive power bleeder to pressurize the reservoir?

    In this rare case, the fluid between the primary and secondary piston is quickly pumped past a seal and up the secondary port into the reservoir. When the secondary piston engages the pedal feels firm and will not sink any further which makes this case hard to diagnose. Only way to know for sure is to put pressure gauges on both outlets--one will have normal pressure and the other will be zero.
    cchief22 likes this.

  18. #17
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    Why wouldn't the volume of fluid in the neck drain back down into the reservoir?
    The screen at the bottom of the filler neck was probably clogged with debris. Brake fluid in the system doesn't materialize out of thin air. Yes, the pressure issue (from a failed master cylinder seal) caused the fluid to route back to the neck...but all else being equal (i.e. clean brake fluid), the reservoir would not have overflowed. That's why overflowing MC reservoirs are not a common occurrence.

    Volume = Length x Length x Length

    Pressure = Force / Length x Length

    People talk about ^both and often confuse the two.

  19. #18
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    Quote Originally Posted by Dan_N View Post
    Except in the one case that does exactly this. A internal seal failure between the two independent hydraulic circuits will cause a loss of braking power to 2 wheels and pressure build up in the reservoir. ALL other diagnostics for testing a master cylinder will pass so almost nobody ever recognizes this failure case.

    Quote Originally Posted by Dan_N View Post
    Where's the vent and how do you plug it when using a Motive power bleeder to pressurize the reservoir?

    In this rare case, the fluid between the primary and secondary piston is quickly pumped past a seal and up the secondary port into the reservoir. When the secondary piston engages the pedal feels firm and will not sink any further which makes this case hard to diagnose. Only way to know for sure is to put pressure gauges on both outlets--one will have normal pressure and the other will be zero.
    Almost all of the above statements are false.


    If there is a master cylinder internal leak:
    1. The fluid in that section will be pumped into the reservoir raising the level no more than a few mms.
    2. Because there is a breather there will be no pressure build up in the reservoir.
    3. There will be a loss of braking power to 2 wheels. (you got that right)
    4. The pedal will go down about half way and become firm, making it very easy to diagnose.


    The vent is in the cap which is not used while using a Motive power bleeder.

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