Billy,
Good news, you have "S" Cam brakes! The brake actuator pushes on the crank arm which twists the shaft that goes through the backing plate. The other end of the shaft has an S-shaped cam between the brake shoes that when rotated via the crank on the other end expands the shoes out to the drum.
The Maxi Brakes are a second actuator piggy backed on the the rear axle actuators. There is a large spring inside this actuator that pushes against the crank arm setting the brakes on a lose of air. When you see an air brake truck pull in to park and then hear a loud burst of air, that's them pulling out the yellow air switch on the dash that dumps the Maxi brake air allowing the springs to set the brakes.
To set your truck up for maxi's you will need the maxi brake actuators and do a little airline work and add the switch to the dash.
As for truck speed, there are on-line calculators that ask for tire diameter (10.00x20 should be about 42-1/4"), transmission final ratio (direct is 1:1, OD could be .8:1) and rpm (DD should be OK at 2000 I think) to give you mph. See what you come up with...
Mike B
