This happens frequently on my CPS 3200 if you use it heavily. You pump out so much water out of the backpack unit, it starts to implode, and not enough air enters the chamber fast enough. You have to wait and allow the air to replace the lost water, and the backpack unit will slowly expand back to its normal shape.
One thing I do on the 3200 is to disconnect the hose, and jam my finger into the entry port to open the check valve, allowing air to flow up the water tube into the backpack to equalize it faster.
On my take, just use it normally, but keep an eye on it. If anything seems to slightly suck in, open the cap and let air in. Usually the caps and the new aquapak units should have valves to take care of this. From what I've seen in my CPS 3200, this should only be a problem if you're using your gun under heavy usage.
"Although it is a beautiful place among the stars, danger lurks in the shadows. That's what Alliance Command's for."
-Captain Ivan Reilly
Closing Speeches After the Xidian Wars