Ok, there is a slight possibility that the fluid being forced back up into the master cylinder damaged one of the internal O-rings. Doesnt happen often, but I have seen this occur on enough occasions to be a concern.
Other possibilities is a twisted brake hose, caliper slightly cocked at an angle when bolted back up, pad sitting at an angle in the caliper, or rotor not flush to the hub, which any of these will cause your problem.
To save time in diagnosing, as well as being a highly accurate way to test, we need to do a brake line clamp test. You will need either 4 brake line clamps, or 4 vise grips for this. If you use vise grips, wrap some old cloth over the ends of them to protect rubber brake hoses.
On all 4 wheels, place a clamp or vise grip about midway up on the flexible brake hose, clamping it gently. Now, test your brake pedal. It should be nice and firm. If the pedal still sinks, then the master cylinder has an internal bypass of one of its o-rings and will need to be replaced. If pedal is good, proceed to remove one clamp at a time, starting at rear brakes, and test pedal after a clamp is removed. At which ever wheel the clamp is removed and the pedal sinks, that is the wheel with the problem. You will need to inspect the hose for kinks or twists, then check rotor placement and pads for being cocked. If all are ok, then the caliper piston is cocked at an angle and will need to be replaced