I had this same problem too and the solution was right here after searching and searching and searching..... needless to say, this one problem hooked me into Steel Soldiers!!!
What I found was that there is a small (dime sized) "O" ring located under a plate screwed into place on the HH (if memory serves me - I think you needed to remove this in order to remove the HH and inspect those "O" rings) and those screws are wired so that they don't back out. Well one of those screws backed out and blew a path through the side of the little "O" ring. I could of stopped here because the damage was obvious, but since I had it this far, I replaced the two "O" rings on the HH anyway. No problems since and it's been about 3-4 years. I thought I took a picture of it, but I can't find it now....