The brake fluid had to go one of three places. You've already checked for an external leak, but there still may be one that is not fast enough to leave a puddle on the ground. Look at the inside sidewalls of the rear tires to see if there's wetness or streaking on them that would indicate a leaking rear wheel cylinder. Brake fluid typically just seeps out of them slowly.
The most elusive place to lose brake fluid is out of the back of the master cylinder. The fluid runs into the power booster, then it gets sucked into the engine where it goes out the tail pipe. To identify that, the mater cylinder has to be unbolted from the power booster, then it must be checked for wetness.
As front brake pads wear, the pistons move out of the calipers to self-adjust. That's how all disc brakes self-adjust. As they do that, brake fluid fills in behind those pistons. That means it is normal for the fluid level in the reservoir to drop. We never top off brake fluid during other routine services such as oil changes because as part of the normal brake job, those pistons have to be pressed back in to make room for the new, thicker pads, and doing so pushes that brake fluid back up into the reservoir. There is enough brake fluid in the reservoir to accommodate the normal brake pad wear, however, if the brake hydraulic system needed a repair in the past, and the fluid wasn't filled sufficiently at that time, it is possible the current low fluid level is simply due to having not enough fluid to start with, and the front pads are worn to the point of needing replacement.
What was the symptom for the "no brakes"? Did the brake pedal go all the way to the floor? Was it as high as normal but just real hard to push? Is there any recent history related to this?
Once the brake fluid was low enough to cause a problem, a lot of do-it-yourselfers think they have to bleed the system at all four wheels to get the air out. In fact, air is not going to go down to the wheels. It keeps on floating back up to the top where it will be expelled by simply pressing the brake pedal a number of times, but to make that go faster, only push the pedal a couple of inches, then release it. Vibration from normal driving will help those few air bubbles work their way out.
Also be aware if the brake pedal got pressed all the way to the floor, there is a real good chance the master cylinder has been damaged. Crud and corrosion build up in the lower halves of the bores where the pistons don't normally travel. Pressing the brake pedal all the way to the floor, whether surprised by a sudden leak, or when improperly pedal-bleeding the system, the rubber lip seals are run over that crud causing them to tear. That results in a slowly-sinking brake pedal, and that often takes two or three days to show up.
This article can help identify a defective master cylinder:
https://www.2carpros.com/articles/brake-pedal-goes-to-the-floor
If you find the master cylinder needs to be replaced, either because it caused the "no brakes", or because it's leaking, or because it got damaged, here's a link to an article on how to do that yourself:
https://www.2carpros.com/articles/how-to-replace-a-brake-master-cylinder
Let me know before you start that procedure, then I'll describe a trick that avoids the need to bleed at the wheels.
When you add brake fluid, be sure to not get the slightest hint of petroleum product mixed in with it. That includes engine oil, transmission fluid, axle grease, penetrating oil, and power steering fluid. Repair for that contamination requires all parts that have rubber parts inside that contact the brake fluid to be replaced. That is a very expensive repair.
Tuesday, February 11th, 2020 AT 11:21 AM
