Hello! Scout FreddyHair back to report.
I found a couple weird bugs with the backpacks -namely, the Cow Backpack; didn't try any other, as of now.
Here we go:
-The hose seems to work weirdly. I collected 4 buckets of lava in one of my tanks and then tried inputting it directly into a Tinker's Construct seared tank. Didn't work as expected: it generated some "ghost lava" (oooo spoooooky), which couldn't smelt anything. The Tinker's tank appeared full, but the backpack one appeared full as well. Then, as I compacted the Smeltery into a Geochest and then opened it again, the ghastly lava was gone (nb: Geochests instead keep any inventory item or tank fluid or thingumajig contained in the blocks they grab). However spilling it on the ground and then collecting it with a bucket appears to work just fine, as does filling the bucket directly in the inventory. In this regard, I also have a question: can you spill milk and melon juice as well?

-When I die, the backpack gets put on the ground in rather weird locations. When I died from a Silverfish nest, couple meter under the surface, my backpack got placed OVER the surface, instead of right next to my grave; something similar happened the second time I died, way farther underground, and I lost it :( luckily there wasn't anything preciousssssss in it. I am not completely sure this second case was a bug in your coding, though: there's a tiny chance that the backpack got instead blown up and destroyed from the TNT zombies (damn DeadlyWorld). I'll have to test this further.

-Apparently it doesn't recognize Tinker's Construct longsword and mattock as tools. Having the pickaxe in my hand, I shift-scroll to the longsword; then it just scrolls through the hotbar, instead of swapping the longsword to the mattock. Same happened with the mattock.

-Every time I place the backpack on the ground it "recrafts" itself. I'll explain further: I get attacked by something with it on my chest slot, it obviously takes damage, and whenever I place it down and grab it up again it hasn't got any more damage. I said "recrafts" because I have the "You Will Die" mod installed -amongst other things, it also gives a random bonus in protection to any crafted armor piece and then changes its name accordingly. Well, if I at first had -let's say- a damaged "Old Cow Backpack" with an armor bonus of 0.5%, when I place it down and grab it up again I could end up with a pristine "Hardened Cow Backpack" with an armor bonus of 3%, just as if I crafted another cow backpack for free. I don't know if that's intentional, but I suppose not.

Oh, another bug I forgot to mention is that I can't break things with the hose in my hand. No animation happens, and grass and whatnot just disappear in front of my eyes; can't break leaves -I can see the "cracks" but when they reach the borders they retract and start again- or anything else. Don't know if that's intentional, tho; I can however assume that grass and whatnot shouldn't just disappear if I try to break it.

Update: apparently it "recrafts" itself also when I right click with it in my hand and put items in it. I tried with a couple items and it did it both times. It also recrafted when I put it in my hand from my armor slot.
Also, the tooltip says both tanks have got a bucket worth of milk, while the right one is actually empty. I got the milk from the ability of the cow backpack, so I could suppose the problem is in that.

Noting all of this.
I'll work on this stuff next week or so. Can you try separating different bugs into different issues? makes tracking easier when I'm fixing it one by one.

I'm new to this bug-reporting stuff, sorry for making it harder. I'll try and behave 0:)

Thanks a bunch!
The cow backpack is indeed weird. Is a side effect of having it nomnomnom wheat and whatnot.
I'll be working on all of this.