I would not think the master cylinder is causing air to get into the system. If this were the cause, then all of your wheels would still have air in them not just the left front.
If you have put a lot of fluid through it (close to a bottle) and you still have air in just this caliper, I would suspect the caliper itself. Also, the fact that you had to replace the bleeders is a tip off as well. I suspect the bleeder itself is not holding or sealing and when you release the pedal and the caliper is releasing, it is pulling air in through the bleeder. If this is the case, replacing the caliper may be the only option. Reason is, even if you get it to bleed the air out, there is no guarantee that when you use the brakes it doesn't start pulling air back in.
The way to confirm this, is to wrap the treads of the bleed with Teflon tape so it seals and then when your partner is release or pumping the brakes, hold your finger over the end of the bleeder so it can't pull air in. If you get the air out, at least you confirmed this is the issue and I would replace the caliper at that point. Reason is, the brake fluid will brake down the tape and if the issue is around the treads it will stop sealing again.
Let me know if you have questions.
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Wednesday, May 15th, 2019 AT 6:53 PM