Bleed brakes -- break screw. Now what?

[shields@tembel.org (Michael Shields)]

Today I went out to my '86 Coupe GT to bleed the brakes, as part of my
general plan to maintain it as the previous owners ought to have done.
So I study the Bentley, lay out all the tools, disconnect the booster,
and pull the RR wheel.  I find the first bleeder screw, put a wrench on
it, and... it shears off in my hand.  It doesn't strip.  It breaks.

So...

1. Most importantly, what do I do now?  How do I get it out?

2. My car is missing the dust caps on the rear bleeder screws (they are
on the fronts).  Could that have caused this?

3. I was using a Craftsman 7 mm box-end wrench.  Should I have been
using something else?

4. Why isn't the screw made from some supermetal?

(And this immediately after replacing the OXS!  Ungrateful car...)
-- 
Shields.