Not new to mechanics but new to a 2004 XJ8 I have just replaced all four rotors and one bad caliper...rear. The rear brakes were done first. I drove the car for a couple days and let it sit. Went to drive out of the driveway yesterday and had instant brake failure. Pumped the brake pedal fast a bunch of times to get it to stop rolling. My immediate thought was either air showed up in the caliper or I had booster problem. So I got my trusty "full time" mechanic buddy to come over and we replaced the front rotors and pads and bleed the front and rear brakes with two bottles of new #4 brake fluid. Got the pedal to pump up somewhat hard without engine running. Started engine and applied brakes and the pedal went to the floor?! Booster? No brake warning lights went on? Car has 72,000 miles on it....looks like new....little old lady drove it 6000 miles in two years? You would think? right?! Please advise.

 

Move your question over to the X350/358 forum.

Look to the master cylinder or booster. Check the brake lines; a good tech would have caught a bulging hose, but you never can tell. Pull all tires and watch each of the calipers while somebody pumps the brakes.

 

If it were me, i would check for leaks and air in the system. Assuming the rest of the brake parts are good (you were in there for pads and rotors, so any leaking caliper would be present at this point), i would venture to guess the brake master cylinder failed.

 

Brake booster failing wouldn't affect the car's ability to stop - it would only affect the force needed to press down on the pedal. Without being able to look at it, assuming there's no leaks, master cylinder seals sound like a good guess.

 

I think there was a TSB for that car relating to brake lines soaking in trapped water due to undercarriage panels. I would think though that if a brake line had corroded through you would see fluid on the ground. But, that came to mind so I thought I would share it.