All professionals know to never push the pedal more than half way to the floor. Crud and corrosion build up in the lower halves of the two bores where the pistons don't normally travel. When the pedal is pushed too far, the lip seals can get ripped on that corrosion. After that, the internal leakage will cause a low and mushy pedal or a slowly sinking pedal when steady pressure is held on it.
There's three times that can happen. When you're surprised by a sudden leak such as a popped rubber flex hose, the pedal will usually get pushed too far. When pedal bleeding with a helper, that helper can push the pedal all the way to the floor, and when just replacing pads, the pedal is able to be pushed too far until the pistons come out far enough for the pads to hit the rotors. For the popped hose, many shops will warn about the possibility of a damaged master cylinder and include a replacement in the repair estimate. Some mechanics even replace them automatically to prevent a problem from showing up days later. If only one lip seal is torn the pressure differential switch should trip and turn on the red brake warning light. Typically if both seals are damaged you won't even have the little braking power you described.
A different problem can occur if you replaced pads on the rear. On models with rear disc brakes, the rear calipers do not self adjust simply by pumping the brake pedal like the front ones do. They adjust by operating the parking brake. Until that is done, the brake pedal will continually be able to be pushed too far. A spring in the master cylinder will provide some resistance to pedal movement and cause some pressurized brake fluid to go to the wheels, but the braking performance will be very poor. Usually both rear calipers are left unadjusted, and since this is a split-diagonal hydraulic system, both circuits do not develop normal pressures so the pressure differential switch and warning light do not turn on.
If any of the cables are broken or rusted tight, you will need to operate the parking brake lever on each caliper by hand. You can usually do that with a large pliers. Once the pistons have been ratcheted out they will continue to self adjust from then on like normal.
If you have rear drum brakes and you replaced the shoes, be sure they're fully adjusted up. If the drums were machined excessively, well beyond the published legal limit, the drums will be a larger diameter than the shoes. That means the shoes will only contact the drums near their center area. Even though it seems like the shoes are adjusted properly, they can pivot on those contact patches and allow the wheel cylinders to keep on moving the tops of the shoes. That will cause a low pedal but it will become firm once the tops of the shoes hit the drums. Again, that's only an issue if the drums were machined way too much. This can sometimes be noticed by observant car owners even when the drums are still legal but the pedal feel will improve once the center area of the shoes has worn down and the curvature of the linings matches the drums.
A less common problem with drum brakes is grooves worn in the backing plates. Each shoe rides on three "lands" or raised surfaces. Grooves worn in them can cause a shoe to catch and either not apply properly or not release smoothly. From your description I don't think that is the problem here but it's worth mentioning. Professionals coat those lands with a special high-temperature brake grease during routine brake service to reduce the formation of those grooves.
I should probably mention too to be sure the calipers are mounted properly. I've read twice here that someone found they had missed the target with a mounting bolt and the caliper was riding up too high. You'd expect the caliper to bang and rub against the inside of the wheel but apparently that isn't always the case. Unfortunately there's no way for us to diagnose that when we can't see it, and that is so uncommon that it doesn't normally get mentioned.
Wednesday, May 30th, 2012 AT 7:02 AM