The cause is impossible to see over a computer, but for the low brake pedal, the most common cause is improper procedures. In particular, the pistons must be pushed back into the calipers to make room for the new, thicker pads. Once everything is reassembled, those pistons have to be pushed back out until the pads contact the rotors. That is done by pumping the brake pedal a few times. Where do-it-yourselfers get in trouble is they push the brake pedal all the way to the floor. Crud and corrosion build up in the lower halves of the two bores where the pistons don't normally travel. Pushing the pedal over half way to the floor, whether bleeding the hydraulic system, you're surprised by a sudden unexpected leak, or simply when running the pistons out of the calipers, results in tearing the rubber lip seals when they run over that crud. That causes a slowly-sinking brake pedal at first, but often not for two or three days, then a total loss of solid pedal.
The fix for this is a rebuilt master cylinder. I have a trick for replacing them that eliminates the need to bleed at the wheels.
There are other causes of low brake pedal, but this is the most common one if only the front brakes were worked on. There are a lot of other things professionals do to prevent noises, brake pulls, and vibrations, and there are a number of things we can do to cause those things if we aren't careful. A lot of beginning mechanics with no formal training make these mistakes too.
Monday, April 11th, 2016 AT 4:52 PM