What's the best way to wash the pack part of a hydration pack (not the resevoir/hose)? Is it just like a back pack: cold water and a little detergent with air/tumble dry low?
Here's what I found from Dakine (brand I have), guess I should've trusted Google first!
DAKINE Pack Washing Instructions
We recommend washing your DAKINE backpack by hand in cold water with a mild detergent and soft bristled brush. If hand washing is not going to do the trick, the second option is to use a "front loader" washing machine. This is better because it has no center post for straps to catch on. There is still a chance the straps could catch so place your pack inside a pillowcase or mesh bag before washing. Wash your pack in cold water with a mild detergent on gentle cycle, Do not bleach your pack. Hang to dry in a warm but shady place, the sun could change the color of your pack. Switching the pack from inside out to right side out throughout the drying process is the best way prevent mildew. Damage caused by washing packs or bags will not be covered by warranty.
As others have mentioned and you've discovered, the safest way to clean it is to simply follow the manufacturer's instructions.
My girlfriend and I both have Camelbaks. Their website says: "DO NOT wash your pack in a washing machine." I've done this with both our bags a whole lot of times and have never had a problem. I've also put a bunch of backpacks through the washing machine that weren't supposed to go there, never with a problem. (Can you tell I'm lazy?) And a hydration pack is basically just a backpack with a special compartment for the bladder, and maybe a bit of extra padding or insulation.
I would, however, recommend hang drying them, no matter how you choose to wash it. Dryers can get hot enough to melt the nylon that these kinds of bags are made out of. Low is probably safe, but why risk it? They easily dry overnight, even indoors.