Q: Front brakes seizing up and over heating

My car had all pads replaced and rotors resurfaced when I bought the car a year ago. Since I've owned the car the steering wheel would shake slightly when braking or occasionally when accelerating. About a month ago I noticed that on and off randomly the steering wheel would shake hard during highway speeds and an audible thud-thud-thud-thud-thud in coordination to the shaking and a burning smell and too-hot-to-touch rim resulted. A week or so later it did it every time at highway speeds. My mechanic resurfaced the front rotors and new pads, no more noise or shake. But three days later the front left brake was seized up with burning smell/too hot to touch and I brought it back to the mechanic who replaced the caliper on the left. That was two weeks ago and today the right brake seized up with smell/hot. I also noticed that the brake fluid was almost to min; I added fluid to the max and pumped the brakes and it did not heat up in the 10 minute drive home. Caliper? Cylinder? Fluid?

My car has 115000 miles.
My car has an automatic transmission.