I've always used ammonia to wash the tanks when done spraying for the year. Use on my 750 gal field sprayer all the way down to the 25 gal ATV sprayer. Cleans out the residue.
I use my yard sprayer to spray roundup on the driveways, rinse with water, wash out with ammonia/water, rinse, then use it for tri-mec on the lawn in fall.