Q:

Brakes Smell Bad and Smoke

Here's the situation:
Initially
Changed a wheel/tire, and snapped a hub stud.   Had to take off the brake caliper on that hub (driver's side).   No problem with that, but then all of a sudden my front end was really jumpy.   It was time to change the pads and rotors for my front disk brakes.   Also, when I put all my weight and force on the brakes it definitely went to the floor with the pedal.   I don't recall this ever happening, though I can't say for sure. 
Brake Change
So then I changed both rotors and all pads for my front disk brakes.   I noticed that my driver side pads had very little pad left (I changed these about 9 months - 12 months ago) and my passenger side had plenty of pad left.   I would say drive side had about 1mm of pad left, and passenger side had 3.5 to 4mm of pad left. 
I also noticed something odd.   When I was trying to compress the caliper piston on the driver's side caliper, it was extremely difficult.   I eventually got it, but it wasn't nearly as easy as the passenger side. 
After the Brake Change
When I immediately drove the car afterwards, the front end shake was gone.   But I noticed something odd.   My driver side wheel has this awful smell, and when I got done driving it (I was hard on the brakes to test out and make sure it was stopping well, and it was) it was lightly smoking at that wheel.   I thought it might have been the brake cleaner just burning off. 
But then a day later, the smoking and smell continued, but stopped later in the day with about 45 miles down. 
Question
What could have been causing this?   Does it sound like my driver's side caliper is just getting stuck, causing it to ride the pads and wear them down?   Is that why it was smoking/smelling? 
Why was the driver's side caliper piston so hard to compress with a C-Clamp?   Could the caliper be going bad? 
Could air in the line cause the caliper to get stuck and/or go to the floor? 
Could this possibly be a master cylinder issue?   But I would think if it was a problem at the master cylinder, it would cause an issue for both front brakes.   Am I correct in saying that the master cylinder has a connection for the front that shares the same fluid/pressure? 
Any ideas are much appreciated. 
Vehicle: 1994 Mercury Villager. 

A:

It sounds like the caliper is sticking.  If the piston is sticking then you would end up with one side worn down more then the other.  Also, a piston should compress with a clamp, if it is that hard then that also points to it sticking.  The smoking then could be from the pads being in constant contact with the rotor (if it is in that strong of contact).  You probably would also notice the vehicle pulling to the side of the sticking caliper.  Air in the line would not cause nor would a bad master cylinder.