Well, the brakes are crappy because the brake fluid has soaked up water vapor to the point where it is compressible. Completely drain and replace the brake fluid. Also, inspect each caliper to be sure none of them have seized. Single piston calipers are more likely to have this problem.
As for the speedometer, I really don't see how it could read low, but if you want you can pull the instrument cluster and run some WD-40 down the speedo cable. Maybe the grease is all dried up and that's keeping it from turning at full speed?
For the rear-end clunk, I would jack up the back of the car and turn the rear wheels. See if you can reproduce the sound and narrow down where it's coming from.
1988 Turbo Convertible
: S4 13BT, T04b V-trim hybrid - 297 whp @ 15 psi
2011 Sonata SE Turbo:
233 whp @ 17 psi
Last edited by RotaryRocket88; 10-23-08 at 04:30 AM.