Brake pedal goes half-way to the floor, only the first time

If you disconnected the caliper while you were working on the front end the system probably needs bleeding - even if you didn't it probably does . Some cars require bleeding both front and rear systems to maintain balance in the valving system . Your repair manual should have the procedure .

No, I didn't. (For others planning to do this: At first I did what I usually do, I hung the caliper from other parts using a bent wire hanger, but later I had a loop of fairly heavy stranded electrical wire and that worked better.)

Is the symptom I have really caused by needing bleeding? I had spongy brakes once after doing something to the brakes, and every time I pushed the pedal it was the same, spongy, like stepping on a sponge. Now it's never spongy. When the pedal is going down the first time, there is almost no resistance (only what comes from the spring) until there is normal resistance.

that is indeed, the text-book symptom of a bad master cylinder.

GW

Given the recent history of the vehicle -IE- it's damage history, I'd be looking at the brake on the damaged axle first. Clamp off the brake flex hose to that wheel. Is the pedal highand firm? If not, look elsewhere. If so, you know where to look. Brake hose clamps are available at Harbor frioeght and many auto parts/tool suppliers at reasonable cost. Some are little screw clamps, others are a vice-grips with round rod jaws. Make your own by brazing a 1/4 inch long 1/4 or