Yup. Gotta determine where that fluid went.
Also, as the front brakes wear, the pistons in the calipers move out to self-adjust, and fluid takes up the space behind them. That will cause the fluid level in the reservoir to go down, but there is enough extra fluid there to prevent it from running dry and drawing in air. Worn front pads should not cause a mushy pedal. When new pads are needed and installed, the pistons are forced back into the calipers and that fluid is pushed back up into the reservoir. There needs to be room for that fluid; that's why we don't top it off during other routine service such as oil changes. If it's over-filled when the brake work is done, it won't hurt anything; it will just make a mess that has to be cleaned up after the extra fluid spills out.
Another thing to look at is the rear drum brakes. If the self-adjusters have stopped working, the shoes will have to move too far before they contact the drums. That too will cause a low brake pedal but there will not be a leak. The clue is the pedal will get higher and harder if you pump it rapidly a few times, then hold it. It will get low again after you release it for a few seconds and apply it again.
Now that you've added fluid, if the level stays up but the pedal is soft, check the rear brakes. If the level goes down noticeably again, there is a leak. If the level goes down slowly, as in it takes a couple of days to notice the change, suspect rear wheel cylinders or the master cylinder. If the fluid goes down after a dozen pedal applications, there is a much faster leak that should be easy to spot under the car. Ruptured rubber hoses, one by each wheel, and rusty steel lines, commonly under the door area, are common causes.
Thursday, May 12th, 2011 AT 4:30 PM