From down the page at:
http://www.flyinmiata.com/techline/olde ... rkey%20dayThe best info Ive gathered on this topic:
.... The Miata's four wheel disc brake system is self adjusting in the front, but requires periodic caliper adjustment in the rear for proper pedal height and parking brake function. At my shop, we always set the rear calipers when doing oil changes and it was usually noticed by the customers.
Anyway, here goes: There is a bolt with a 14mm head on the back of each rear caliper. Actually there are two bolts of this size, one holds the bracket for the handbrake, one unscrews to gain access to the caliper adjuster. Pull the one covering the adjuster (don't worry, no fluid should come out!) and you will find an allen type adjusting screw. It takes a 4mm wrench to turn it. The technique is to (with the handbrake off) turn the adjuster in (clockwise) while rotating the brake rotor by hand. As soon as it stops or you feel a heavy drag, stop and back the adjuster off 1/3 turn. Make sure that the rotor now turns freely. Reinstall the 14mm bolt and repeat the procedure on the other side.
And the article goes on to describe the solution *If* your allen bolt strips.....
Peter