The brake fluid overflowed because an uninformed individual filled the reservoir previously. Brake fluid needs to be added when there's a leak that has been repaired; no other time. A lot of car owners get angry when their mechanic does not top off the brake fluid during other routine service such as oil changes. In fact, if the brake fluid is low, where did it go? We will never add brake fluid unless we did work on the system.
Professionals know that brake fluid level goes down as the disc brake pads wear. The pistons move out of the calipers to take up the space, and brake fluid fills in behind them. That is how all disc brakes self-adjust, and is why the level in the reservoir goes down over time when there is no leak. When you stick new, thicker pads in, you have to push the pistons back into the caliper housings, and that is what pushes the brake fluid back up into the reservoir. If someone filled the reservoir in the meantime, the fluid is going to overflow and make a mess. Brake fluid eats paint too, so be sure to wash it off right away.
Mechanics will check the brake fluid level during an oil change. If it is low, they will first look for signs of a leak while the vehicle is up on a hoist. Next, they will try to peek through the wheels to see if the pads are worn. If they are, or if they cannot tell, they will add "brake inspection" to their list of recommended services.
There is no need to even think about bleeding the system. You have too much brake fluid. There is no way air is going to get into the system from simply replacing rotors and pads. What is more important is that you do not get any grease on the friction surfaces. If you do, it must be cleaned off with brake parts cleaner before any of the parts get hot from normal braking. Once they get hot, any grease will soak in since cast iron and brake linings are porous. That contamination will never come out and will cause squealing.
The piston and caliper surfaces that contact the pad backing plates must be cleaned with a flat file to insure no dirt or other debris prevents the pads from sitting squarely. Those contact points, and the points where the caliper sits on its mounting surface, must be lubricated with special high-temperature brake grease. That reduces the transmission and amplification of squeals and other noises, and it reduces the tendency for grooves to wear in that pads can catch on later. The edges of the pads should be ground off slightly if they didn't already come that way, to prevent squealing. If the squealing can be prevented during the break-in period, they won't squeal later. Calipers must never be allowed to hang by the rubber flex hoses as that can tear the inside lining of the hose. Calipers should be hung by wire or supported in some way. As you can see, there's a real lot more to a brake job than just hanging new parts on the car.
Most importantly, when adding brake fluid, be absolutely sure to not get any hint of petroleum product in there. Less than one drop of engine oil, transmission fluid, power steering fluid, or axle grease can result in repairs that cost more than the value of the car. Brake system specialists even wash their hands to remove fingerprint grease before handling brake parts that contact brake fluid.
Thursday, April 28th, 2016 AT 3:00 PM