Brake fluid cap vented?

I'm narrowing my brake issues down and it seems to be fluid expansion related. Fluid isn't even a year old and I've been having the issue before and after the fluid change. Does anyone know if the brake fluid cap is vented for when the fluid expands to allow air out? I removed some fluid and it still does it but not as bad. I can't see the fluid level after I put the cap and sensor in so I can judge where I am with it sealed up.


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Yes, it has a vent to let air in to displace the fluid as it goes down during normal wear of the brake linings.

Some master cylinders have a little rubber device on the bottom of the cap that expands as the fluid goes down. Same principle though.

Does it let the pressure out as well? Where on the cap is this too so I can make sure I'm not clogged.


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There shouldn't be any pressure, other than atmospheric that is, in the fluid reservoir. The rubber flap that oilhammer described is a check valve which allows air in, but not out.

Don't know. But I know if I drive a couple miles in the city no major traffic the fluid level goes up. I would think if the air pressure isn't vented it may cause a couple psi of pressure. Minimal but it seems to be causing some drag. I just took out some more fluid trying to get the level to have enough expansion room and will find out tomorrow during the peak heat of the day.


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Seeing that much change in the fluid may indicate a different problem.

BobnOH said:

Seeing that much change in the fluid may indicate a different problem.

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What could it be? Short of one caliper the whole system is new. I've made many posts and nobody ever gives anything to try that hasn't already been done so I have just been throwing parts lately.


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A small increase in level with temp is expected, but not much. About all that could drive it up a lot is a dragging brake, boiling the fluid in that caliper/cylinder, and pushing the fluid back up the line. You would, typically, get a very soft pedal along with that though .
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tadawson said:

A small increase in level with temp is expected, but not much. About all that could drive it up a lot is a dragging brake, boiling the fluid in that caliper/cylinder, and pushing the fluid back up the line. You would, typically, get a very soft pedal along with that though .
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The drag is extremely light. It looks like I'm still above the max with the cap installed even before it warms up so I'm trying reducing fluid more and more and it does seem to last longer without the drag. After it gets dark and drops into the high 70s it never drags so it's barely at that point. Maybe the dragging calipers that I've since replaced did boil the fluid a little at some point. A good bit of the fluid has been replaced since but if it doesn't solve this way maybe I'll do another full flush.


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Still getting a little drag. Guess tomorrow I'm gonna try to see if I can see the level with the cap and all on with the flashlight. Any suggestions on being able to see it?


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Decided to get a fluid tester. See if maybe my fluid is just saturated somehow because of how it's been open for a couple hose changes and such.


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You are looking for a ghost that does not exist. More likely your problem is dragging E-Brake cables.

Ol'Rattler said:

You are looking for a ghost that does not exist. More likely your problem is dragging E-Brake cables.

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I've disconnected them and made sure the mechs were both at the stop. Brakes are smooth and drag free until they are used for a little bit and the fluid gets heat in it. Lifting the brake pedal let's it disengage but the lift isn't even measurable. Have done other posts going thru the whole process I've done over the last 10 months but I've never gotten any other things to check out. I've actually had 2 mechanics who have said they wouldn't even look at it based on what I've changed because they wouldn't even know where to start. All I know is that when it's dragging if I pull the cap off the reservoir it immediately releases.

Searching I've read posts of BMW,Nissan, and many motorcycles that have a vent to vent air out the reservoir when fluid warms up and expands. Can't find anything on vw though.


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did you do what I mentioned in the other post, shimming the M/C away from the servo a bit to guarantee release/clearance?

Is it ALL wheels or one/a pair? Have you checked for kinked or deteriorated flexys? Do you ever feel the ABS cut in unnecessarily?

It's not the ABS. Drove for a bit with the fuse pulled just to make sure. I've only ever had the abs kick in once for me. All 6 of the flex lines have been replaced.

It is all 4 wheels now, had single wheels dragging more in the past but I got that solved with other parts. I haven't tried the shimming thing as I can't find a shim setup to try on it. I will assume the seal needs to make good contact to properly seal. Can you direct me to somewhere I can get the proper shim?



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The rubber diaphragm, between the cap and the reservoir, found on most brake master cylinders, is impermeable, however it does expand and contract via bellows and cap vent as the fluid expands and contracts and the fluid level changes... this is so moisture laden atmospheric air never touches the brake fluid during normal operation

jimbote said:

The rubber diaphragm between the cap and the reservoir on most brake reservoirs is impermeable, however it does expand and contract via bellows as the fluid expands and contracts and the fluid level changes... this is so moisture laden atmospheric air never touches the brake fluid during normal operation

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On some cars yes, but not on Volkswagens (at least none that I know of, and I've worked on some dating back to the late '50s).

The OP's car has a vent in the cap, little tiny hole on the underside of the level sensor connector "stalk" that vents directly into the cylinder in which the level sensor plunger rides.

Easy to check, just take the cap off and blow through it.

The A5 cars use a little vent hole with side channels in the spot where a rubber seal is, the channels are there to prevent the seal from covering the vent hole.

oilhammer said:

On some cars yes, but not on Volkswagens (at least none that I know of, and I've worked on some dating back to the late '50s).

The OP's car has a vent in the cap, little tiny hole on the underside of the level sensor connector "stalk" that vents directly into the cylinder in which the level sensor plunger rides.

Easy to check, just take the cap off and blow through it.

The A5 cars use a little vent hole with side channels in the spot where a rubber seal is, the channels are there to prevent the seal from covering the vent hole.

Click to expand...

Any idea on where to get a shim for our cars?


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a couple of bits of card would do, it doesn't have to seal.

heck, just loosen the fasteners a turn after the car is hot and draggy and see if they release.

Problem found. Rear pedal mounting point is broken and the bolt isn't holding.


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Scratch that. Apparently the bracket is supposed to look broken and not connected. Vw dealer confirmed this.


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