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(happened a few days ago while driving) My brake fluid went from high to low. The e-brake light on dashboard would light up. I refilled the resevoir to full while installing fuel filter(car on jacks for a few days). The next day the resevoir is empty. I checked for leaks:
-inside, on pedals and carpets...no sign
-all 4 calipers and wheels...no sign
-brake cylinger, booster, lines running out from it....no sign
Can't find where or why brake fluid is/are leaking. Is this a common problem. In in the process of swapping mk3 brake cylinder because of this.
Also follow all the lines from master to calipers.Rust will get the brake lines reight in front of the drivers door,quite often also.
My guess is rsdeo nailed it,booster is probably filling up and needs changed along with master.