[Lynn Eades]

This is from memory and guessing, so if no one else responds try this. Bolt your two plates to the floor, I think you will be able to calculate the spacer widths. I'm guessing 3/4" dia. (19mm) Make a 3/4, and a 5/8 spacer, and see how that goes.
The knob on your brake lever is held on the rod by a small pin.
To gain access to it, you will need to turn the pall around, so that you can push the rod back up the handle as far as you can.
They are tricky because the rod binds inside the handle.
If you remove the knob, that allows you to remove the spring. If the spring is not rusted, and its seat is ok, then the problem is the rod is binding in the handle. dont try to pull it out, just try to improove it where it is. (I think the lever had its bend put in it after it was assembled) There are a few NOS items around that are like yours.
Cheers.