I am dealing with the exact thing. My girl's bumblefoot has been going on since February.
You need to do daily soaks in Epsom Salts to draw the infection toward the skin and keep it out of the bones and joints. After soaking for 5 or 10 mins in this warm epsom bath, inspect the foot. If the scab is black, or yellow and puss colored, you will need to reopen it and drain it again. I have done major surgery on my birds foot 5 times in the last 10 weeks. I have recently changed the regime, and what I have been doing is each day, I pry open the wound and squeeze and dig around as each day I am getting more gunk. You will not always get that "kernel" or "core" of pus out of there. I didn't find a "kernel" until the 5th time I went into the foot.
Once the infection has been in the foot long enough, all the surrounding tissue has become infected. So a daily drain is needed. After you dig around and drain it that day, use some iodine on, and then pack it with neosporin. Then bandage it up. Repeat every day until it is healed. At some point after a week or so has gone by, you can let the wound rest and see if it clears up. But be prepared to go back in.
As far as antibiotics. Amoxicillin or Penicillin works on Bumblefoot. I was using Amoxicillin at first with my girl, 250 mg a day split up into 2 oral doses of 125 mg in the morning and then again at night. (disolve 125 mgs in 2cc's of water)
However my girl has taken this infection too far, and now I have started her on Penicillin. Injectable. You can get that at most feed stores. The Amoxicillin you can give orally down the beak with a syringe, (without the needle) I am using human grade Amox, however you can also use the "fish" type of Amox on chickens. I have used it when I don't have the human stuff on hand.
Unfortunately with bumblefoot, antibiotics alone will not clear this up. You also have to phsyically remove the infection.
Good luck with your bumblefoot case. I am right there with you!!!