Not my car, but I’m working on a ‘08 Lincoln MKZ with brake issues. It’s got me thoroughly confused.
The car was driving totally fine, then one day the brake pedal was suddenly quite soft compared to the day before. It stopped as per usual, but pedal travel was mostly to the floor before it would bite.
I pulled the wheels off to check for leaks & the condition of the pads/rotors. No exterior leaks anywhere, reservoir was full, and the pads were nearly new from a not-too-long-ago brake job (done at a shop).
I changed the brake fluid since the old stuff was basically black, & everything went smoothly. No issues bleeding the system. The pedal felt a bit firmer after that, but still not what is should be.
I did some online research & thought that the master cylinder might be to blame, so I put in a new one. The pedal felt almost normal immediately after that. Took it out for a drive & did some aggressive braking to test it out. It wasn’t great, but pretty darn good. It stopped when you wanted it to, so I wasn’t too mad.
Now we get to the confusing part. The next day, the pedal was back to going almost all the way to the floor. It would stop, but not safe to drive. I thought maybe I should try bleeding the calipers to make sure there wasn’t air in there from doing the master cylinder, but the damn things won’t bleed now!
Here’s how it’s going: the car is up on jacks & has ZERO brakes. When I open the bleeder valve, fluid drips out as it should. When I put the tube of my one-man-bleeder over it, it slowly starts filling with fluid. Some air bubbles come out too. When I go & push on the brake pedal, it looses pressure after say 3 pumps. This is while the car is OFF. Some fluid flows into the tube when my foot is down, but it flows right back into the caliper when I lift off. Each pump pushes less & less fluid out of the calipers.
I tried the two man procedure too, opening & closing the valve while someone else works the pedal. It keeps the fluid from flowing back in, but no fluid really comes out either.
So, I guess there’s air somewhere. Maybe quite a lot of it. I thought it could be in the ABS system, so I put the car in drive & got the rotors spinning, then slammed the brakes to activate the ABS & hopefully move things around. I’m not kidding when I say there’s no brakes, I had to put her in neutral and wait for the rotors to stop spinning before putting it back in park! The pedal went to the floor & felt like it was grinding something. It was horrible. Not a normal ABS buck-off feeling.
I did this twice, then tried bleeding again. No change. I attacked the ABS for a 3rd time, then it flipped out. Suddenly the ABS light came on. Scanned it & got two codes, C1235 (right rear wheel speed sensor) & U0415 (invalid data from ABS module).
The ABS shut itself off, there are still no brakes, and it still isn’t bleeding properly. I did try to bleed it once more before I quit today, and it was a little better than it’s been, but the fluid is still flowing up & down not really making forward progress.
I don’t know what the hell is wrong with this thing. Could I have done something horribly wrong when I put the new master on? Even if that’s what’s causing this issue, I don’t get why it went from perfectly fine to having a ton of pedal travel overnight in the first place!
Any thoughts would be appreciated. This car always seems to have some major (expensive) issue, & it’s driving me nutty.