Heater Valve Issues
 

I have a 92 caprice 305 wagon, and having a really strange problem with my heater valve. A couple of months back, I somehow broke my heater valve (just old, I guess). When I bought the replacement, it lasted about a day before it started spewing coolant out of the vent hole on the outside of the casing. Figuring this was a defective part, I had it exchanged. That one lasted a while before developing the same issue. I've now just gone through my third replacement and am kind of getting tired of this. The Haynes manual doesn't even mention how to change this part, but it's only 3 fluid hoses and 1 vacuum hose, so it's kind of hard to get it wrong. I'm either missing something really obvious here, or having a long string of bad luck. Does anyone have any idea what could be causing this issue? I'd appreciate any answers or kicks in the right direction.
Thanks.