I discovered this neat trick an hour before setting off on a climbing holiday after finding that my CamelBak hose was disgustingly dirty.
Being loathe to spend £20 for a brush on a wire to clean the hose and without time to get one I looked around for something that could clean the hose.
Paracord was the answer! A loose knot in the end of a length of paracord, dropped through the hydration bladder's hose can be drawn through slowly and bring all the gunk with it.
Here's how I've done it ever since.
Step 1: You will need....
Step 2: Soak
This softens up all the dried on gunk growing on the inside of the tube.
Don't pour the water away once the hose is soaked, save it for when you're finished cleaning the hose with the paracord then you can steralise the paracord before drying and storing it.
Step 3: Dry
Step 4: Knot and drop!
Drop the non-knotted end down through the hose until it comes out the other end. Make sure you have a neat end on the paracord or it won't make it all the way down the tube! If it gets stuck try holding the end of the paracord in the tube and pulling the length yet to be inserted. This straightens out the cord and helps it slip down. Sucking on the end also helps draw it through.
Move the knot up to the end of the tube and slowly tighten it until you can just push it inside the tube. It should be as a tight a fit as possible. I can feel the bulge of the knot being pulled through the tube when I run it through.
Step 5: Pull it through
Slowly draw the knot through the tube by pulling on the other end of the paracord. When it's almost all the way through hold it over the sink; if you've never done this before you'll likely have a lot of black gunk dropping out on your lap otherwise! Mine's cleaned regularly so I can't show you how gross it can get.
Wash off the knot and repeat until you're happy. If the knot wasn't touching all sides of the tube the first pass through you might be able to see stripes down the inside to show the difference between before and after cleaning.
Once clean, rinse and then it's ready to use.