Why does my 2004 trailblazer wheels lock up?

Calipers are old and worn ? They are a good system but operate in the worst conditions , There's nothing to retract them they merely relax . When I do the brakes I peel back the piston boot and shoot some penetration fluid in there before I compress the piston . Been doing it for decades and never had to replaced calipers . You'll see other cars with discolored wheel covers from the brake dust created by calipers that don't relax . Others may disagree but that's my story and I'm sticking to it .

Can you drive for a while then they start to lock up? If so then these is too much moisture in the brake fluid, the fluid heats up as you drive turning the moisture into steam and setting the brakes, once the fluid cools it turns back to simple moisture and the brakes release. Flush the entire brake system. I have also seen the antilock system mess up and apply the brakes on its own, try electrically disconnecting the unit and see if the problem goes away. Now this will return the brakes to normal braking, so there will be no anti lock.