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Hi all,
I have a 1980 e12 520. The left front brake is not quite seized but it is sticking. The wheel doesn’t freely spin but takes some force. I took a look at the pads and rotors and no abnormal wear. Do I have a bad caliper piston or should I check brake lines first? I bled the brakes and lines are getting fluid but not sure how consistent or fast fluid should be expelling when I bleed them. Any advise would be helpful. I don’t want to replace the caliper just to have the problem persist

Well, if your lines are old I would definitely recommend replacing them and you might as well pull your calipers off to give the pistons a through check. Maybe use compressed air with something in between the pistons to force the pistons out and ensure they are all moving smoothly and equally.
Also check your wheel bearing. It might be bad, in need of greasing, or the hub nut might be too tight. Always a good idea to pull the wheel hub apart and clean and re-grease the wheel bearings. Then ensure proper tightening of the hub nut upon reassembly. If the nut is too tight it can cause excessive drag on the wheel and make it harder to spin.

A trick to get the pistons out of the calipers : Remove the calipers from their mounting brackets while still connected to the lines. Free of the rotors, pumping the pedal will allow the pistons to leave the cylinders easier than dealing with compressed air in the line connectors.

Should any of the pistons remain stubborn, be obviously careful to protect them from damage using channel locks, etc.

I’m curious, would this work?: dismount only the sticky caliper, leaving the free caliper on the rotor to constrain the free piston. Shouldn’t this result in more available system pressure to push on the sticky piston? It should at least provide more time to work the sticky piston out before the free one would advance too far from its caliper if it were dismounted. The free caliper could next be removed and the piston advanced, on the theory that the sticky piston will remain stationary enough.

The calipers, pistons are basically very simple. Remember the Dot 4 fluid attracts water over time, to cause the sticking, light rust, aggravation. That said, rebuilding the system is not that terrible. More an exercise in patience dealing with the fluid's excellent paint removing qualities.

Of course you can do as you wish, one each, whatever works.

My car had the original flexible brake lines on it. You can tell because there will be a ring with the year on it in my case "79". I read that they can look okay from the outside but collapse on the inside so I replaced mine. It's a no regrets move, definitely firmer pedal feel and you'll bleed the whole system anyway.
I agree with Champie though check your wheel bearings too. But it does sound like one of the pistons is sticking and as Mr Hanson said, rebuilding them is not an ordeal. I need to do it myself, my car pulls a little under heavy braking.