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Did a front brake job on my Q and noted that the brake master cylinder reservior tank has two chambers - the front chamber is about a third of the size as the main chamber. Also, there is a small plastic "knob" on the side of the front chamber. During and after purging the old fluid, I noted that the front chamber still contained the old, dark fluid and did not purge like the main chamber.

What is the front chamber used for? Is it an overflow chamber? Is the knob on the side a drain for that chamber? Does it twist or pull or _____?

Several mechanics, as well as parts counter personnel in the area have no idea. Alldatadiy was no help either.

Help?
 

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I've never noticed that on my own car but I haven't had to change my brake fluid out yet, if I was to take a guess it's likely a dedicated fluid is for the ABS system or overflow chamber as you suggested
Let me check the SM though and see what it says, I'll update my post with the findings.

Edit:

Hmmm I don't actually see anything in the SM that covers the reservoir itself in any great detail, I'm starting to lean
towards the fact it's an over-flow chamber and you just need to bleed off the old brake fluid.

I'm sure someone else will chime in here, but if not maybe give the dealership a call and have them confirm.
 

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What is Alldatadiy?
 

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Alldatadiy.com. It’s a paid online service manual site. It’s OK but not very detailed. Sure beats a Chilton, etc., though.
 

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What is Alldatadiy?
Alldata is an online database that non-dealer shops subscribe to to access service information. It appears Alldatadiy is the same type of site but designed for the non-professional do-it-yourselfer. It used to be our local libraries had a subscription to Alldata but the last time I used it, the information available at no charge was rather limited.
 

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I've never noticed that on my own car but I haven't had to change my brake fluid out yet, if I was to take a guess it's likely a dedicated fluid is for the ABS system or overflow chamber as you suggested
Let me check the SM though and see what it says, I'll update my post with the findings.

Edit:

Hmmm I don't actually see anything in the SM that covers the reservoir itself in any great detail, I'm starting to lean
towards the fact it's an over-flow chamber and you just need to bleed off the old brake fluid.

I'm sure someone else will chime in here, but if not maybe give the dealership a call and have them confirm.
My understanding of the brake fluid reservoir design is that there are two sides divided by a baffle plate, one side is for the fluid going to the master cylinder and the other is the return from the ABS actuator and electric control unit. The brake fluid level can fluctuate during operation and exceed the level of the baffle plate. Excess brake fluid spills over into the overflow chamber of the reservoir in order to maintain the proper brake fluid level in the main reservoir.

It's not for dedicated brake fluid for the ABS system based on the design diagrams of the braking system I'm looking at. The output from the master cylinder goes exclusively to the ABS actuator and electric control unit.
 

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My understanding of the brake fluid reservoir design is that there are two sides divided by a baffle plate, one side is for the fluid going to the master cylinder and the other is the return from the ABS actuator and electric control unit. The brake fluid level can fluctuate during operation and exceed the level of the baffle plate. Excess brake fluid spills over into the overflow chamber of the reservoir in order to maintain the proper brake fluid level in the main reservoir.

It's not for dedicated brake fluid for the ABS system based on the design diagrams of the braking system I'm looking at. The output from the master cylinder goes exclusively to the ABS actuator and electric control unit.
So if I understand your explanation correctly, one would just remove the old fluid from this "overflow chamber" and not replace it with fresh fluid? And are you saying there are three chambers total?
 

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So if I understand your explanation correctly, one would just remove the old fluid from this "overflow chamber" and not replace it with fresh fluid? And are you saying there are three chambers total?
I haven't drained my reservoir so I can't see the internals to verify the existence of a inlet/outlet baffle plate. qw50adix says there isn't one so this particular design evidently doesn't use it.

I would think one would drain old brake fluid from the "overflow chamber" in addition to the fluid in the main reservoir but I'm not sure when refilling with new brake fluid that the "overflow chamber" would be filled also. I don't think so though.
 

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Drained my brake fluid last spring and didn't notice there were two chambers.
Aye he has a 2014 though, so it's possible G37 had the multi-chamber system (I didn't notice) or it may have been unique to
that year or just Q50's with the VQ37 engine.
 

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Aye he has a 2014 though, so it's possible G37 had the multi-chamber system (I didn't notice) or it may have been unique to
that year or just Q50's with the VQ37 engine.

I just checked the part number and it seems the same for all MY including the 2.0L engine and Q60:


  • SKU: 46091-4GK5A
  • Other Names: Reservoir Assembly, Tank Assembly-Brake Oil Reservoir
  • Description: Q60. Q50. 3.7L. 2.0L, 3.0L.
 

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I just checked the part number and it seems the same for all MY including the 2.0L engine and Q60:


  • SKU: 46091-4GK5A
  • Other Names: Reservoir Assembly, Tank Assembly-Brake Oil Reservoir
  • Description: Q60. Q50. 3.7L. 2.0L, 3.0L.
Huh aftermarket res or wrong part installed at some point perhaps? Or maybe we are all just blind. Hah
 

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Maybe the latter lol we're probably replacing the fluid too soon that's still clean and we're unable to discern the difference between empty and filled chambers.
Hah probably, I'm changing out the fluid in my G soon as well and I'll keep the an eye out.
 

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There is a baffle in the brake fluid reservoir. Maybe the baffle is there to stop the ABS module from sucking air when the brake fluid is low?

What I normally do is suck out the old brake fluid using a brake air bleeder rubber tube. If the tube is small enough you can move the tube around to suck out 90% of the oil brake fluid. Fill the brake fluid reservoir with new brake fluid and do the normal bleeding of the brakes at each corner.

If you have the sports brakes there are 2 brake nipples per caliper that needs to be bleed. I didn't know about this until I turn the steering wheel full lock and notice the other brake nipple on the back side of the brake caliper.

https://www.google.com/search?q=brake+air+bleeder&safe=off&sxsrf=ACYBGNSBwGW_fUALeidb_krGCT5YuErzCQ:1568287646365&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjku8TglsvkAhU273MBHU5MDLUQ_AUIESgB&biw=1843&bih=1159
 
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