Tie a piece of cloth to one end of a piece of string, the cloth sized to fit through the tubing very tightly, the string a foot or two longer than the tubing, and a knot tied on the tag end.
After your good tubing soak, with hot water running through the tubing, feed the knotted end of the string into the tubing, the water running through will pull the string all of the way through if you keep the string feeding into the tube freely. Put the cleanser of your choice on the cloth and pull it through. You’ll be amazed at how well you can clean your tubing. Repeat as much as you like.
Alternately, you can shove a wet piece of cloth with cleaner on it into one end, and attach that end to an air compressor. Slowly increase the air pressure until the cloth is pushed through the tubing. This is more fun, and I’ve blasted cloth through the tubing a bit dangerously, and it’s quite messy. The string method is also more thorough.