That is what should happen with a normally working dual piston caliper. Instead of loosening the bleed screw and introducing air to the system, what you should do is put a piece of soft wood across the two caliper pistons and then compress them evenly together with a 4”~ish C-clamp.

Have you bled the whole system properly? If you don’t have a helper or vacuum system, do a gravity bleed over a long time, and possibly twice.

Do you have an ABS light on? Because the most common things to cause a single brake locking to the degree you are describing would be a bad caliper/piston or an ABS fault.

Next I would service the caliper slide pins. I am told the hats on these are marginal and if you are in a salt zone, that could be the problem. Hopefully you stripped them and re-greased them when you had the caliper off, and checked the hats or used new ones.
Other possibilities are a collapsed line, which it sounds like you addressed.
Contaminated fluid can cause it.
A bad master cylinder can do it - but it should also affect the rear wheel on the opposite side, too. Check that wheel for excessive pad wear, or just if the hub is hot also like the front. If you find that to be the case, suspect the master. If the wheels lock, and you unbolt the master cylinder and slide it forward a little and they release, then you have a strong clue there.