If you have no "free travel" in your clutch lever , the washer trick is not going to change things . Your master cylinder is going to give you X amount of fluid for the slave to move. If you add too much thickness the slave piston will bottom in the slave cylinder and will maintain constant pressure on the release mechanism. The metal surfaces the friction material contacts has to be perfect. If the discs are larger or smaller in circumference you will have a problem . Think of the o/s edge of a 33 1/3 LP . I hope you can . If the metal surface looks like this , the friction material will drag here and cause you problems.