You can adjust them without pulling the tires off. Put the axle on stands, just bearly getting the tires off the ground. On the back side of the brake housing at the bottom there is a rubber plug that gains access to the adjuster.
The adjusting nut on the cables that you have turned all the way down now, needs to be turned back towards the center of the screw before you turn the adjusters.
Use a flashlight and you can see the adjuster. If you don't have a brake tool you can use a regular screwdriver to turn the adjuster by the teeth.
There is a flat locking arm that is spring loaded and rest on the teeth to keep the adjuster from turning in by itself. You may or may not have to use another screwdriver to push against this arm to turn the adjuster.
You want to turn it up a few clicks, then spend the tires. Do this till the wheels start to not turn freely and are slightly dragging. Now turn the adjuster a few clicks down and you are done.
Now adjust the nut on the cables till there is tension about halfway up as you pull the handle.
Also, have you looked at the bracket on the handle? There are two bolts that holds it to bracket on the tub. I had the front bolt and welded nut strip out, making it lift up as I tried to set the brake. I drilled out the nut to 1/4" and used a longer bolt and nut to fix this.