Another brake problem--antilock system malfunction?

Hello all,
2006 Tacoma base, 49,000 miles, V6 automatic, air, no other factory options; no traction control. I just replaced the front brake pads because the second aftermarket NAPA master cylinder (of 3) activated only the front brakes. A few days ago I stopped momentarily to get the mail; a normal, gentle stop. I tried to go, the front brakes are locked. I stepped on the brake pedal, the brakes released. I tried the brakes several times going up the driveway; once the brakes locked again, tight enough to hold the truck on a ~7% slope and cause a rear wheel to spin when trying to go. Stepped on the brake pedal, the brakes released. The locked brakes happened once again while running an errand, and released when I stepped on the brake pedal again.
The inboard pistons on both sides were hard to push back into the calipers, but once pushed back in, the pistons moved freely; i.e., push one in, the other one in that caliper moved out. Before this locking problem, I had jacked up each front side and spun the wheels by hand; there was no sign of the brake pads hanging up.
I'm mystified. A friend suggested that maybe the antilock system was malfunctioning and occasionally not releasing the brakes until the pedal is depressed again. Does anyone have any thoughts or suggestions?

 

When they lock up you need to start cracking brake lines loose at each component starting at the master cylinder and working out toward the wheel until they unlock, once they do the component where you loosened the line at is having an issue.