Over the last several months, I was annoyed by a continuous buildup of water on the bottom of the reefer. At first I attributed it to condensation. I live aboard, so it's always on, but recently I had to take the battery charger to the manufacturer, so I emptied the reefer and turned it off. That's when I realized that the water was coming from the galley drain (and thus I also figured out why the water was so murky :(). Sure enough, I took the backflow valve out, tested it, and it wasn't blocking backflow anymore. I bought a new one, and I'll install it today.
But now my question is: shouldn't the foot pump be blocking backflow, too? (Of course, the valve is there for a reason, and if it's broken, that certainly doesn't help.)
BTW, the Shurflo valve I had does not seem to be available anymore (part #140), so I got part #340 instead, which looks completely different, but should do the job.
I had the same problem. Talking with the parts guy, he thought a new valve might last 3-4 years. I don't remember the price he quoted me, but I thought it was way out of line. Found one at Lowes in the well department that fit right in. It was under $10 for the valve and hose nipples. Sure makes a difference in the bottom of the refer. :)
Fulvio
If you haven't already, I suggest you check out the Projects section on the website, Ice Box & Galley Drain. I contributed a shutoff valve article that addresses your question. It's only one of a number of ways to do this but it works for us.
Stu