Changed front pads on an 01-05 Honda Civic. Now the pedal can be pushed almost to the floor when I'm at a stop, but feels good when driving.
Should I replace the rear pads first before I do further diagnosis?
Most likely cause of pedal softness (and I don't mean poor braking, I just mean pedal feel/position).
No, thickness differences between front/rear pads (or between any of the sets of brake pads, even between the inside/outside of a single rotor) will make no difference on pedal feel on a floating-caliper setup like used in your Civic -- assuming that all the other braking system parts are in good shape, i.e. rotors are true, calipers are free to slide and pistons aren't frozen, etc.
Pedal softness is typically caused by improperly bled brakes, a failing master cylinder, or some other issue with the hydraulic system.
Rear pads take less wear than the front, so don't need changing at the same time - change them when they're worn (always change both sides when worn, though).
The real issue is the pedal being pushed to the floor. That sounds like air in the system (or even worse, a leak). Bleed the brakes, and see if it goes away.
