Besides replacing the master cylinder, what else did the last mechanic to your car do? If there really were metal filings in the brake lines, then the entire brake system should have been purged of the contaminated fluid. A proper (and expensive) job would involve flushing the lines and disassembling or replacing the calipers. It would be a lot cheaper, and probably acceptable to flush about two-thirds of the fluid in each line (a guessing game in itself) and inspect the fluid. If there were absolutely no metal in the fluid, it might be adequate to flush the remaining fluid out and risk not removing the calipers.

If the mechanic did all of this, even minus the calipers being replaced, then $600 seems reasonable. Of course, if this all were done properly, then there would not be air in the system again.

Since there is air in the system again, I would start asking questions. Were there filings in the master cylinder itself? Was the entire system flushed completely? Was the fluid checked? Did the mechanic determine how the air got into the system (i.e., was it a bad master cylinder which caused both problems)?

At the very least, you should not pay the last shop any more money until it can tell you how air is still getting into the system. If the answer involves the master cylinder or getting the remaining air out of the brake lines, they should fix it free. If a different component is broken, you have to ask them and yourself if this was the original problem--or at least did this problem exist when you had the master cylinder replaced?

-Optimist