You need to have a better look at the brakes. The quick check you performed doesn't tell you much about how they work in the real world with real world forces applied.
Some of the things to check with the caliper are torn piston boots, how evenly the pistons can be force back on one side compared to the other, the condition of the caliper slide pins and the condition of their lubrication.
Some other brake related things to check are the condition of the lining, the condition of the rotors, confirm proper lubrication of the pad backing plate where they slide on the caliper, and the condition of the hoses. A restricted hose will cause a pull to the opposite side.
Also check the suspension. In particular the lower control arm bushings. They have to be really sloppy to cause this, but others have reported worn bushings to be the cause of brake pull.