I don't think the booster is the problem here. You've done a lot to the system, including (from your description) bleeding the brakes while the caliper is off the car (????).
The last step in working on brakes is the bleeding.
You need to take a close look at the brake resevoir. The resevoir is designed with a partition, this partition combined with the location of the resevoir makes it hard to discern if the rear chamber is full or dry. Often, when bleeding the brakes, the rear chamber goes dry which defeats the effort.
A good rule of thumb is to top off the resevoir after every third crack of the bleeder valve if bleeding by the two-person method. Might be easier to keep the resevoir full by building your own pressure bleeder. Do a search on "Pressure bleeder" and you'll find directions on constructing your own.
If not comfortable with, or knowledgeable in, the brake system, this might be a job better left to the pro's.
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