I have a 2004 5 cylinder XC90. The brakes have been getting worse and worse to the point where Tonight I backed out of the driveway and had no brakes at all. The pressure sensors are reading low. No fluid is leaking anywhere and there is no hissing noises present. The brake light is not on. What all do I need to replace?

 

Possibly the brake booster, but you be the judge. Brakes are pretty simple. The ABS stuff is passive outside of 1 or 2 seconds a month.

My cars are all 5 cylinders, and I have seen a couple of occasions where it was not running under very good vacuum and power brake suffered. I have also noticed that the booster hose is really hard and you can't really remove and reinstall it without at least some concern that it won't seal back. Getting vacuum to the booster might be an issue, but usually they are pretty reliable of course.

 

did the pedal go to the floor or did you simply have to use two feet to get any traction on the brakes? If you loose your vacuum boost you will have to push really hard on the pedal but the pedal will be firm. If it builds pressure after a short drive (say 100 yards), then your booster is fine but you may have either an issue with the vacuum pump (which kicks in when the car first starts up ) or the brake booster lines/hoses. If it stays off all the time, try listening for a whoosh when pressing the pedal which could indicate a torn diaphram on the booster. If your pedal has a normal feel but can slide down to the floor or can pump up to build pressure, then you may have a leak internal to the master cylinder. As noted the ABS system is out of the loop for the most part - its job is to toggle the pressure going to the wheels when it senses a lock up.