Some people use Diesel fuel. I recommend a 50/50 mix of mineral spirits and lacquer thinner.
Go to Lowes or Home Depot and buy 1 gallon of each along with an empty 5 gallon bucket/lid. It will help break it up and doesn't harm the rubber hose and pump seals.
Pour both cans into the bucket. Pull the stick pump out of the ISO and have it cycle a couple strokes to prevent so much ISO draining into your clean mix.
Your proportioner is on jog or cycle - NOT AT 1000PSI - just recycle and you can use your hose heat. Hold the pump into the bucket and start to cycle it. Allow the ISO to drain back into the drum. Don't drain it into your clean solvent.When the fluid starts to sputter, that's when the solvent is starting to come out.
I recommend putting some pantie hose on the discharge side of the hose to catch the garbage so you don't continually flush junk back into the lines.With the hose heat on, and pumps on recycle, have a person work each part of the hose by bending it to help break up some of the ISO inside.
Continue to flush the lines until you are satisfied with the discharge. I usually run 5-10 minutes. DO NOT allow the mix to sit in the hose for several hours. This can soften the o-ring seals if left in for a long period of time.
This is only a flushing agent.To store the lines for winter, fill them with hydraulic oil from a tractor supply house and some people use Diesel fuel. Either one is fine - the idea is to fill the lines and keep air out. My farm tractor has had hydraulic fluid in the lines for 55 years and it hasn't hurt them.
Repeat the process to go back to ISO by draining the lines back into your bucket until you notice the ISO starting to flow