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The brake fluid reservoir level drops very fast in my 2007 4WD V6 Toyota Tacoma, probably within 3-5 days of filling it up. The funny thing is, it drops to just below where it says minimum but not further down, which makes me exclude the possibility of brake lines leaking somewhere downstream in the system and suspect that the problem is that the reservoir itself has a leak very near the top which causes this initial level drop. The brakes work fine but this causes the brake light to be on all the time.

I was wondering if replacing the reservoir will involve draining and bleeding out the whole system. Or can I just pump some out the top to below where the reservoir is mounted, replace and refill it?

EDIT: I noticed that the master cylinder fluid tank is mounted onto a drumlike device (what is this called?), which, I assume transitions and adapts the pedal motion to the cylinder. Does the liquid from the tank go into that drum as well? Because if it does, it could be that the drum is leaking somewhere.

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    Do you see any fluid leaking somewhere? That fluid need to go somewhere. Check where the brake pedal rod go through the firewall from inside the cab, if it's leaking from there it could be a master cylinder failure. Also, I've read that some brake fluid leaks to the booster, but it's pretty rare.
    – Gabriel Mongeon
    Oct 21, 2013 at 14:37
  • @GabrielMongeon, haven't observed leakage but I could try. But if it leaked where you say it might, wouldn't it keep dropping even below the minimum notch and not stop there?
    – amphibient
    Oct 21, 2013 at 14:39
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    that seems a pretty odd behavior. The thing is that all that brake fluid have to go somewhere, once you find that you'll be able to better diagnose the issue.
    – Gabriel Mongeon
    Oct 21, 2013 at 14:58
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    This may be silly but also check for stains on the floor after long parking periods. And I wouldn't completely discard a leakage down the line, since pressure in fluids increases the deeper you go, it COULD just be (though would be much of a coincidence) that the needed pressure to go through a crack down the system is found with oil above your minimum mark.
    – PedroC88
    Oct 22, 2013 at 0:59
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    Another possibility is that you just have a lot of air in the system (failure to properly bleed it). This happened to me - brakes were "mushy" and I kept "losing" fluid with no visible leak. Eventually, after enough top-offs, it stopped happening and the brakes got firm.
    – R.. GitHub STOP HELPING ICE
    Oct 22, 2013 at 3:08

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As another pointed out, your master cylinder could be faulty. I had mine resleeved with stainless steel. Any brake & clutch specialists like repco can do this. To remove and replace is not complex.

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    Yes, it was the clutch master cylinder that had a leak (I suppose some inside gasket blew as a result of age/mileage) so I replaced it. It was a good learning experience as I got to bleed the system, i.e. get it to be bubble free
    – amphibient
    Jun 9, 2015 at 2:02
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I just had same issue on Corolla 2006. Dropped brake fluid fast, but master cylinder area dry. The spot on driveway where it would pool was under master cylinder. It ended up being the right caliper, then leaking along frame upper side and escaping coincidentally under the cylinder. I've done lots of repairs in the driveway, this was hard to identify without a lift.

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