2009 Nissan Versa Brake Problems

If the piston fell out of the caliper, either improper procedures were followed that allowed that to happen, or the brakes were grinding metal-on-metal for a long time and ignored, and the rotor wore down beyond the legal limit. Regardless, there's air in that caliper that needs to be bled out. Simply adding more fluid won't solve anything. The left and right front brake calipers are on different hydraulic systems, so if the left caliper isn't working properly due to that air in it, only the right one will be working. Most manufacturers have the suspension system geometry redesigned to offset that pull when one brake isn't working. You won't notice anything on Chrysler products. On most other brands all you might see is a very slight wiggle in the steering wheel when the brakes are applied. You shouldn't be getting a hard pull.

Also, if the brake pedal was pushed to the floor at any time, either before, during, or after the brake service, it is possible the master cylinder was damaged, not from improper procedures, but from the crud and corrosion that normally build up in the lower halves of the bores where the pistons don't normally travel. Pushing the pedal more than about half-way to the floor runs the lip seals over that crud and can rip them. That applies to any vehicle more than about a year old.